Abstract: According to Cycle by Cycle calculation procedures, the range of stress intensity ΔK along the length and depth direction of surface cracks under cantilever bend loading respectively to build the relationship between crack length and depth, and the crack growth rate was calculated by using the material constant C, m of 10 CrNiMo high strength steel and the Newman-Raju formula and Paris formula. The possible error of theory calculated value was analyzed and Newman-Raju formula was modified.The results showed that the trend of surface crack growth and crack growth rate calculated were consistent with the test results. the modified Newman-Raju formula could agree well with the experimental data fitting curve both in crack growth shape and growth rate. Simultaneously, surface crack growth could be well forecasted with different initial crack shape of sample using the modified Newman-Raju formula.