Historically, mathematical literature and textbooks thought that the Pythagorean School was the first to discover and prove that √2 is not a rational number. This paper will reveal that the Pythagorean School’s proof that √2 is not a rational number is invalid, That is to say, the discovery of the first irrational number is based on the invalid proof that√2 is not a rational number, so it’s very essential to rectify the statements about the first math crisis in the history of mathematics.