An online monitoring device is designed for the real-time monitoring of hard X-ray beams at Shanghai Synchrotron Radiation Facility (SSRF). Compared with the existing beam position monitors and gas ionization chambers, it can monitor the beam position, size, and flux, and reconstruct the relative phase variation of the incident wavefront with an acquisition speed of 2 kHz, without affecting the downstream experiments. In addition, it has potential applicability for feedbacks of the optical devices and experimental positions in super-long and micro-nano-focused beamline stations. Experiments are carried out in the USAXS and micro-focusing SAXS experimental stations at SSRF, and beam stability data are collected at common experimental frame rates. The time-frequency domain signals of position, flux, and size distribution of the incident beam are analyzed, and the phase variation of the incident wavefront is obtained as well. Finally, the feasibility of this device is verified.