A system for preventing signal quality degradation in a CDMA system includes a phase locking unit for controlling a phase of an oscillating frequency signal by controlling a voltage control oscillation unit, a mixer for mixing a frequency oscillation signal and a radio frequency signal received through an antenna, a demodulator for demodulating a signal output from the mixer, a signal detection unit for detecting a signal demodulated by the demodulator, a comparison unit for calculating a constellation and an error vector of the signal detected by the signal detection unit and comparing the constellation and error vector to a setup reference signal, and a controller means for controlling the phase locking unit, mixer and demodulator selectively in accordance with a comparison result of the comparison unit so as to make the constellation and error vector of the signal coincide with the reference signal. The system, thus, prevents the degradation of signal quality and call disconnection occurring during calling.