The pupils, from Andoversford Primary School in Cheltenham, called the test "incredibly stressful", "difficult" and "uninspiring".
The letter was shared on the class' blog on 20 May and offered Morgan suggestions on how to improve the next SATs reading papers.
"We are deeply disappointed by the poor quality of the texts included in this year’s SATs reading test," the letter read.
"As Year 6 pupils, we are expected to learn and demonstrate the skills that your department have set us. However, we do not feel that the reading test reflected these high standards."
Pointing out the issues, the children mentioned most sentences in the paper began with the word "the", rather than fronted adverbials, whiche they had been taught to use to make texts more interesting to read.
They also mentioned there was "limited complex punctuation" - for instance, there were no semi colons or ellipses in the paper.
"The subject of the third text was already especially dull, but in addition the layout was appalling and was a nuisance to read," the pupils continued.
"We were amazed that there was not a single sub-heading. Without these - or other ways of guiding the reader such as bullet points, arrows or numbers - it was almost impossible to find any specific facts or information."
Their final criticism was the "confusing and old-fashioned language" that was used.
They came up with a list of words they were unfamiliar with, including rehabilitate, sedately, parched, skittishly and heavenwards.
"We’re sure that not many adults use these in their everyday conversations," they wrote.
"We are also positive that it was not only us who found this confusing; this would have made the questions very difficult to answer for many children around the country. Is this really fair?"
The pupils said as 600,000 children would have taken the test, it was important the Department of Education understood how they felt about it.
"After this harsh test, we felt our self-belief melting away," they wrote.
"We hope your department takes our advice seriously so next year’s Year 6s will not have to suffer in the same way that we did."
People who have seen the open letter posted online have commented and congratulated the children for writing it.
"What a wonderful heartfelt letter," one woman wrote.
"Your teacher must be very proud of you and has done a great job of teaching an inspiring group of young adults. You all have bright futures ahead of you."
Another commented: "Spot on. My Year 6 class in Swindon could not have put it better themselves. Well done Andoversford. Your writing definitely meets - if not exceeds - the national requirement."
Commenting on the letter, a Department for Education spokesman told The Huffington Post UK: "We are grateful to these pupils for taking the time to write to us and appreciate their feedback on this year’s primary school tests.
"We want to make sure children are learning to read, write and add up early on, so they can thrive in school and working life, and tests are an important part of this.
"But they certainly should not be a cause of stress for pupils - they are there to help teachers make sure children are mastering key skills, and we know many schools manage them well.
"We trial new tests with Year 6 children and take on board both their feedback and comments from teachers.
"This year’s reading texts were chosen following positive feedback from children during trials but we welcome the comments of these pupils and will take them into consideration when developing future test materials."