In order to get the characteristics of optical field in the pigtail fiber of fiber coupled laser diode and increase the pump efficiency of fiber laser,propagation ray calculation and experiments of transmission efficiency in bending pigtail fiber are presented. In bending fiber experiments,it is found that more than 15% of output power differences are given by fiber coupled laser diode with the same nominal output power and pigtail fiber. It means optic modes need to be chosen so as to maintain the pumping efficiency and stability in pumping application. Based on the analysis of propagation ray theory,a new approach to judge the beam quality of fiber coupled laser diode by bending fiber is presented. It also can be used in detecting the fiber coupled efficiency of laser diode.