According to the working principle of a certain aviation turboshaft engine starting process, a novel algorithm is proposed to estimate the 'steady-state fuel flow' of starting process by combining with test data of this engine and merging rotor dynamics and numerical simulation. Based on this algorithm, a new method is presented to establish a mathematical model of turboshaft engine starting process, and the corresponding computer program is prepared and the simulation results are compared with the engine test data. The results show that the numerical results given by the created mathematical model of starting process are of good agreement with test data. The proposed mathematical modeling method of the turboshaft engine starting process is of some versatility.