An online detection system is developed for measuring the longitudinal velocity spectrum, flux and populations in specific states of an atomic beam, through the fluorescence and the time of flight (TOF) method. A fluorescence collection optical system with a plug and play and focal length adjustable mechanical structure is designed to improve the system integration and the signal to noise ratio (SNR). The system is used to measure and analyze on-line performance of an atomic beam, with a program based on LabView for the control of the photomultiplier tubes (PMT), the timing of TOF, the sweep range of the probe laser, and smoothing the results. The experimental results show that the SNR up to 571 (in 20 ms) can be obtained in this system.