The utility model provides a hydraulic control system of a sewage suction truck, which is characterized in that a port A1 of a control valve I is communicated with a rod cavity of a lifting oil cylinder, a port B1 of the control valve I is communicated with a port A3 of a one-way balance valve, and a port B3 of the one-way balance valve is communicated with a rodless cavity of the lifting oil cylinder; an A2 port of the second control valve is communicated with an A port, the A port is communicated with an A5 port of the one-way throttle valve, and meanwhile the A port is communicated with a V2 port of the two-way hydraulic lock. A port B5 of the one-way throttle valve is communicated with a rodless cavity of the rear cover oil cylinder, and a port C2 of the two-way hydraulic lock is communicated with a rod cavity of the locking oil cylinder; a port B2 of the control valve II is communicated with a port B, the port B is communicated with a rod cavity of the rear cover oil cylinder, andmeanwhile, the port B is communicated with a port A7 of the sequence valve. The beneficial effects of the utility model are that through the combined control of the control valve, the inductive switch and the sequence valve, the sequence action of automatic opening and closing of the rear cover is realized, and the labor intensity is reduced.