In this paper, seismic quality factor Q estimation from vertical seismic profile (VSP) data is discussed, by using continuous wavelet transform (CWT). We suppose that source signature is a general constant-phase wavelet which matches the real one better. Based on the CWT of a reference and a target recording, we derive the formula of frequency-independent Q estimation by the ratio of wavelet-domain peak amplitude of these two recordings. Wavelet-domain peak amplitude denotes the peak module of CWT of the recording with every fixed scale. The formula is related to the dominant frequency and standard deviation of source signature. And the Q estimation formula with impulse source can be considered as its specific case that the standard deviation of source approaches the infinity. The synthetic zero-offset VSP and real field tests demonstrate that in the condition of constant-phase source signature, estimated Q from our formula can provide more accurate information than that by the formula with impulse source.