Abstract: With the increase of renewable power integration, the modular multilevel converter based high voltage direct current(MMC-HVDC) with grid-following control has gained more importance in the transmission system. The transient stability of MMC-HVDC will affect the secure operation of power system. The transient stability mechanism of grid-following MMC-HVDC is studied. The dynamic model of phase-locked loop(PLL) is developed first. Firstly, the grid-connected dynamic model of MMC with phase-locked loop(PLL) as synchronization unit is established. Then based on the model, the influence of grid strength, fault-ride through control and PLL parameters on the transient stability is studied. The grid-connected transient stability of the MMC enhances when the power grid strength is high and the reactive component of injection current during the fault increases. Reducing the proportion and integral gain of the PLL is beneficial to transient stability. According to the stability mechanism, a control strategy of injection current is proposed for the MMC to improve the transient stability by matching the current angle with the grid impedance angle. The time-domain simulation in PSCAD/EMTDC verified the conclusions and the effectiveness of the proposed enhancement control.