The invention provides an on-line automatic sampling device for lubrication oil of an engine test bench. The on-line automatic sampling device comprises an oil conduit, a solenoid valve, a sampler, a fixing support and a controller, wherein the solenoid valve is installed at the tail end of the oil conduit, and the oil output pipe of the solenoid valve is inserted into the sampler; the sampler is fixed on the fixing support, and comprises a sampling barrel of which the upper end is opened, a rotary table center shaft which can be rotationally and vertically fixed, a rotary table fixed on the rotary table center shaft, oil sample bottles fixed on the rotary table and a transmission gear and a small motor in transmission connection with the rotary table; the small motor and the solenoid valve are both in signal connection with the controller. According to the on-line automatic sampling device for the lubrication oil of the engine test bench disclosed by the invention, continuous sampling of suspicious oil sample operating points is realized through automatic rotation of the rotary table, the entire device is placed in a test room, the sampling is remotely controlled through the controller, and deceleration and load reduction of an engine are not required while sampling to improve the safety of sampling personnel.