The structure of PDH technique is greatly simplified when the fiber components especially fiber electro-optic modulator (EOM) are applied to it. Since the key of this locking technique is frequency modulation, unaligned modulation process induces residual amplitude modulation (RAM), which produces a temperature dependent direct current (DC) offset and a lineshape distortion. This effect may induce the drifting of frequency locking point and shortening of locking time. A theoretical and experimental analysis of PDH error signal including RAM is given, and the symmetric PDH error signal of RAM reducing method based on Hnsch-Couillaud technique is obtained experimentally. The locking performance of the laser to the cavity is analyzed with and without RAM reducing feedback, and it is verified that the RAM reducing scheme greatly increases the stability of fiber EOM based PDH frequency locking technique.