Abstract: The influence of expander performance on compressed air energy storage (CAES) system has been widely studied. However, only a few researches focus on the effect of irreversible loss, and especially the influence of pressure loss on expander performance is often neglected. The addition of lubrication oil has a marked influence on the pressure loss. To clarify the influence of the lubrication oil on pressure loss of single screw expander (SSE), a comparative experiment was undertaken to investigate six different viscosity lubrication oils (10#, 22#, 32#, 46#, 68#, and 80#) under different operating conditions. Results show that pressure loss, proportion (Δp_in, Δp_out, PLR_in, and PLR_out), and pressure energy loss influence rate of shaft efficiency (PELR_e) can be significantly reduced by increasing kinematic viscosity of lubrication oil. In contrast, although the shaft efficiency can be effectively improved to a certain extent by increasing kinematic viscosity of lubrication oil, there is no positive relationship between them.