The invention discloses a variable-displacement blade type oil pump comprising a pump body and a pump cover arranged on the pump body. A sliding valve is arranged in the pump body in a sliding manner.An adjustment spring is arranged between the sliding valve and the pump body in a compressed manner. An impeller drive shaft is rotationally arranged in the sliding valve in a penetrating manner. Oneend of a mechanical valve elastically extends out of the pump body and abuts against the sliding valve. Movement of the sliding valve drives the mechanical valve to slide into or out of a cavity, sothat the mechanical valve opens or closes an oil discharge channel, and therefore machine oil in a second pressure adjustment cavity can be decompressed. The variable-displacement blade type oil pumpis simple in structure and sensitive in adjustment, through a mechanical type decompression device, the mechanical valve is made to open or close the oil discharge channel so as to flexibly conduct decompression on the machine oil in the second pressure adjustment cavity, and the machine oil is prevented from being blocked in the second pressure adjustment cavity.