The invention belongs to the technical field of petroleum drilling, particularly discloses a drilling fluid chemical-osmotic pressure difference testing device, and further provides a testing method applied to the drilling fluid chemical-osmotic pressure difference testing device. The drilling fluid chemical-osmotic pressure difference testing device is designed for simulating and measuring the osmotic pressure difference of formation water in a drilling fluid and mud shale. The drilling fluid chemical-osmotic pressure difference testing device comprises a rock core clamping device and a mud rock core barrel arranged in the rock core clamping device; a mud rock core is placed in the mud rock core barrel, and the mud rock core divides a cavity of the rock core clamping device into a first cavity and a second cavity; the first cavity is connected with a drilling fluid tank and a vacuum pump, and the second cavity is connected to the vacuum pump and a formation water tank; and the first cavity is provided with a first pressure detecting device, and the second cavity is provided with a second pressure detecting device. According to the provided drilling fluid chemical-osmotic pressuredifference testing device and the testing method, the chemical-osmotic pressure difference of the drilling fluid and a formation clearance fluid can be effectively simulated and measured, and reliabledata support is provided for improving drilling fluid inhibition and the well wall stable capability.