Based on a delayed mutually coupled system consisting of two semiconductor lasers (SL) with different injection currents, the influences of the asymmetric bias currents of two SLs and the frequency detuning Δf (Δf=f1-f2, where f1 and f2 are the free frequencies of SL1 and SL2 respectively) on synchronization performance have been investigated experimentally. The results show that for the case of the two SLs with identical free oscillation frequencies, the mutually coupled system can achieve excellent chaos synchronization under relatively large asymmetrical injection currents. Furthermore, the frequency detuning, controlled by adjusting the temperature of one of the two SLs, has an obvious influence on synchronization performance. For the case of the SL1 biased at a relatively much larger current than that of SL2, the synchronization performance will degrade with the increase of the positive frequency detuning (f1>f2), while the synchronization performance can be further improved with suitable negative frequency detuning. The simulated results are basically consistent with experimental results.