The invention relates to a true triaxial stress-based coal and rock mass creep mechanical behavior testing device which comprises a pressure chamber, four horizontal stress loading oil cylinders and a vertical stress loading oil cylinder, and a long-term loaded oil cylinder steady-pressure oil supply system; four horizontal stress loading holes are formed in the side walls of the pressure chamber, a vertical stress loading hole is formed in a bottom board, a test sample loading and unloading through hole and a detachable type sealed cover board are arranged on a top board; stress loading pressure levers are arranged in the stress loading holes; pressure heads are arranged on the inner end of the pressure levers and the lower surface of the sealed cover board, a stress sensor is arranged between the stress loading pressure lever and an oil cylinder piston rod; a rigid connecting tube is arranged between an oil cylinder body and the outer wall of the pressure chamber, and a grating sensor is arranged between the pressure lever and the rigid connecting tube; a plurality of groups of pressure heads are arranged according to different sizes; and the long-term loaded oil cylinder steady-pressure oil supply system consists of an oil tank, an oil pump, an one-way valve, an overflow valve, three electromagnetic changeover valves, three steady-pressure tanks, three pressure compensation valves and three throttling valves, and all the oil cylinders are supplied with oil through the long-term loaded oil cylinder steady-pressure oil supply system.