We demonstrate an experimental observation of coherent population trapping-Ramsey interference in cold 87Rb atoms by employing the time-domain separated oscillatory fields method. The interference fringe with line width of 80Hz is obtained. We propose a novel method to measure the cold atom number. The measurement is insensitive to the pump beam intensity, the single photon detuning and even the initial state population. We use this method to normalize the interference signal and to improve the signal-to-noise ratio significantly.