In consideration of phase drift and intercept-resend (I-R) attack, the quantum key distribution (QKD) system based on double Mach-Zehnder interferometers is analyzed, the model of detectors′ input signal is built, and the formulas are presented to describe the relationship between the phase drift angle and quantum bit error rate (QBER), as well as the drift angle and eavesdropping information. Meanwhile, a possible method to increase the key generation rate is proposed. Analysis shows that phase drift causes extra errors and damages the system stability. Compared with I-R attack under ideal conditions, eavesdropping information declines, thus the eavesdropping information estimated by privacy amplification could decrease and the final key generation rate would increase. Regardless of the relative phase drift in transmission fiber, QBER varies with phase drift as cosine function, whose period decreases by half under total I-R attack, meaning more sensitive to phase drift. When eavesdropper chooses to attack 55% of all keys, she can gain 25.5% information undiscovered.