The pressure process line at typical sections of a siphon outlet was measured by physical experiment. Pressure pulsation of the siphoning formation process was obtained by the neighborhood average method, and the statistical characteristics and short-time Fourier transform signal processing method were conducted to systematically analyze the universal change law for time-averaged pressure and pressure pulsation. The results show that the time-averaged pressure on the wall, along the flow direction, first decreases and then increases. The time-averaged pressure on the upper wall of the hump segment changes with the flow rate and is gradually greater than that of the lower wall. The pressure pulsation in the process of siphon formation has a positive skew distribution, and the quadratic mean deviation keeps decreasing along the direction of the flow. The pressure pulsation in the process of siphon formation is composed of the rotation of the pump and the movement of water-air, which has a frequency between 0 and 30.Hz, with the dominant frequency close to the rotation frequency. Compared to the entrainment stage and siphoning flow stage, a significant increase in pressure frequency and amplitude occurs in the purging stage, which is close to the natural frequency of pumping station, leading to structural vibration. Design and operation should be optimized to reduce the duration time of the purging stage.