In order to investigate and improve the transmission characteristic of the promote gearbox used in jack-up offshore platforms, the structure and the transmission principle of the gearbox was analyzed. And then the mesh model based on MASTA was established to study the mesh characteristics of the gearbox. With the contact ratio taken into consideration, the macro geometry parameters for the transmission system was optimized and the effect of contact ration on the contact characteristics was analyzed.The results show that the p-p values of time varying transmission error of the last three stages gear pairs are too large in both normal jacking and pre-load jacking conditions before optimization; by comparison, both the maximum contact stress of the contact pair and the p-p value of time varying transmission error of each gear pair decrease after optimization, the mean value of mesh stiffness and the transmission capability of the gearbox are also improved.