The effects of injection locking on the timing-jitter noise properties of a passively mode locked semiconductor laser are investigated theoretically via a set of simplified soliton perturbation equations. We find that, with injection coupling coefficient of 10-3 ps-1, the timing-jitter noise of the slave mode-locked laser can be reduced from a few picoseconds (3.83 ps) to a few femtoseconds in the frequency range from 100 kHz to 10 GHz. We also discuss the impacts of the steady state phase difference between the injection and slave pulses, coupling coefficient, and linewidth enhancement factor on the timing-jitter noise. Calculations show that timing jitter is insensitive to the phase difference while coupling coefficient has a strong effect on the timing jitter, and a smaller linewidth enhancement factor leads to better timing jitter performance.