Based on correlate technique conditions of marine gearbox，it is proposed to take run out value of input and output couplings as the stiffness evaluation of marine gearbox. This new method is put forth on rated load by considering the deformation interrelationship in transmission of some main parts，such as shaft，bearing，gear and box. Finite element analysis of strength and deformation of marine gearbox under forward and backward conditions are conducted. Parameters，such as box and axis deformation，bearing and shaft block stiffness，etc. are obtained and run out value of input and output couplings is calculated. Experiments are carried out at gear drive test bench to verify the reliable of stiffness evaluation method. Compared with the experimental results，the error is in 11 %，which shows the proposed stiffness method is valid.