Abstract: Under cantilever bending loading, pertinence of surface crack length 2c to displacement of loaded point of the specimen was investigated, which was controlled by cyclic load with five notched smooth plate specimens. The results showed that there was no correlation between 2c and displacement valley-value of loaded point, but the correlation between 2c and displacement peak-value DPof loaded point is very obvious. During the original stage of surface crack propagation, DPkeeps nearly a constant and with further propagation of surface crack, DPbegins to increase and at the last stage of surface crack propagation, DPincreases obviously. At the median and last stage, 2c and DPsatisfies a relation of polynomial increased by degrees. The function relation of 2c and DPwould provide reference for measuring surface crack length automatically under the conditions of complicated circumstance.