A measuring method of transverse mode in a laser beam is presented, which is based on the spatial distribution of single pulse energy in a pulsed laser. An algorithm for this method is designed and proved to be reliable in theory and experiments. The spatial distribution curve of single pulse energy is derived by numerical simulation for Qswitched diode pumped solid state laser. The transverse mode proportion calculated by this method is exactly equal to those calculated directly by numerical simulation. An experiment is also conducted to measure transverse mode proportion by using this new measuring method. Spatial distribution of single pulse energy is obtained by testing single pulse energy at different positions in different planes. The transverse mode proportion in Qswitched diode pumped solid state laser is calculated by forming a system of equations with those testing data which is consonant with the spatial distribution curves of single pulse energy fitted by the calculated proportion and transverse mode distribution in those different planes.