It has been mathematically proved that pi is an irrational number, mathematics has infinitesimal but there is a minimum in physics. The Planck length is the smallest length that can be measured, and a size smaller than it doesn't make sense. By com-paring the circumference of a circle with the Planck length, the significant decimal places of the circumference of the circle are determined, with formula: Pi = circum-ference / diameter, calculate the number of significant decimal places for pi. Therefore, pi is a finite decimal and is a rational number, according to this, set up the physical pi table. In the same way we get: The square root of 2 is a finite decimal and is a rational number, and has an exact place on the real number line, resolved the square root crisis of 2. This paper argues that, mathematics and physics are different, Irrational numbers are all rational numbers in physics. There is infinity (n→±∞, n→±1/∞) in mathematics, but not in physics; Length, quality and time all have definite values. Our universe is certain and limited.