Transmission efficiency is a critical characteristic that determines the overall capability of planetary roller screw mechanism (PRSM). This paper addresses the limitations of the traditional transmission efficiency model of the PRSM, which does not take into account the effects of machining errors, complex conditions and lubrication factors. Firstly, machining errors are incorporated into the load distribution calculation model. Subsequently, based on contact lubrication theory, the stress distribution, lubrication viscosity, and oil film thickness in the contact area of the thread profile are calculated. Then, by combining the load distribution model that considers errors with the friction torque model that takes lubrication into account, a new transmission efficiency model has been developed. Analyze the influence of factors such as load, rotational speed, machining errors, and lubricant viscosity on the transmission efficiency. Finally, transmission efficiency tests were performed on the PRSM. The relative error between the experimental and theoretical values was found to be within 6.31%, which significantly confirms the accuracy of the proposed transmission efficiency model.