A study has been made to find out a new method of calculating the survival rate of a fish population from length composition and growth equation. 1. In the steady state of the fish population, let the total mortality rate be z, the age of complete recruitment $alpha$, and the number of $chi$ year class $N_chi$. Then ire obtain $$Nchi=Nalpha;exp;{-z(chi-alpha)}$$ Let the oldest age in the catch be h, the average age between the age of complete recruitment and the oldest age in the catch $Uchi$. Then we have $$Uchi=frac{a-b;exp;(-z(b-a))}{1-exp;(-z(b-a))}+frac{1}{z}....(1)$$ and then let be infinite. Then we obtain $$Z=frac{1}{Uchi-alpha....(2)$$ 2. Calculating numerical value of $Uchi$ from age composition table and growth equation and substitute in (1) or (2) for it, we may obtain the value of s and $varrho^{-z}$. 3. This method is applied t a case of yellow croaker in the Yellow Sea and the East China Sea. The results are as follows: Total mortality rate 0.82595 Survival rate 0.43782 95 percent confidence interval 0.43767-0.43797.