This paper deals with the power loss evaluation in gearshift assistant mechanism applied in automated manual transmission. The originalities include building the numerical models of gearshift assistant mechanism efficiency analysis for cost penalty evaluation and designing gear ratio matching algorithm to match the gear ratios of epicyclic mechanism with desired ones, which is expected to minimize the clutch friction losses by reducing the angular velocity differences between two clutch pads. In the numerical models, gear meshing losses and friction losses are considered the main power sink. By the kinematic relationships and power flow analysis, a set of generalized efficiency calculating formulas of simple epicyclic mechanism in six working modes with respect to teeth numbers are summarized, which are useful computer-aided tools in calculating multiple simple epicyclic mechanisms’ efficiency. The matched gear ratios and the efficiency models are tested on a powertrain simulation model and several simulation results are reported. Finally, this paper summarized the merits and further research targets of the proposed transmission, which is a promising structure to achieve swift and smooth gearshift with low power losses.