Now I submit a manuscript named “Did the Pythagoreans prove that √2 is not rational?” The reason for submission is: the first mathematical crisis is an important event in the history of mathematics, However, over 2500 years, its expression in all mathematical references (including textbooks) does not conform to historical facts. This paper will reveal that the Pythagorean School failed to prove that √2 is not a rational number, because this proof is wrong about the setting of Counter-thesis,Moreover, the application of reductio ad absurdum is unreasonable,