To improve the adaptability and computational efficiency of hybrid DC transmission system models, an electromechanical transient modeling method for a hybrid DC transmission system is proposed based on the idea of current source. The current source models of a line commutated converter (LCC) and a voltage source converter (VSC) are derived from their quasi-steady state models, and the general current source model of a DC grid is formed based on the differential equations of DC branches. A multi-rate hybrid integration algorithm is used to solve the AC and DC power systems, and the numerical problems in special cases, e. g., LCC reverse current, breaker closing and opening, are solved by combining integration step rollback and integral method switching. The above model is developed in a domestic general electromechanical transient simulation program. Simulation analysis on the Baihetan-Jiangsu hybrid HVDC transmission system is compared with an electromagnetic transient simulation. It verifies the effectiveness and accuracy of the proposed method, which well balances simulation accuracy and calculation speed, and provides a powerful tool to improve the simulation ability for a large-scale hybrid AC/DC power grid. This work is supported by the Beijing Natural Science Foundation (No. 3172015).