In this article, the influences of the length of SFL on the measurement of the shape of Fizeau interferometer is discussed. The 3-D diagram of the square difference of the nearest interferometric stripe radius changing with the length of SFL and the radius of measured surface, and the 2-D curve between the difference and the length at R→∞ are given. On the other hand, the relationship between the measured range and the length of SFL and the measured precision is also given. At last, through experiment the coinsidence between experimental and theoretical result is demonstrated.