The mode characteristcs for vertical-cavity surface-emitting lasers (VCSEL) with different cap layer thickness are numerically analyzed. The results show that the threshold gain can increase several times for VCSEL with unmatched cap layer, which results from an anti-phase reflectivity from the air-interface. The mode field intensity distribution inversely increases near the air interface as the reflectivity of the air interface is anti-phase with that of distributed Bragg reflector. A precision etching of the cap layer can be used for improving the laser performance.