The invention relates to a pile soil interaction mechanism testing device for a riser, which comprises a sealed cylinder, a sealed cover, an electric piston hoisting machine force transducer, a testing conduit, a plurality of strain force transducers, a liquid pressure gauge, a hydraulic pump and a control device, wherein the electric piston hoisting machine force transducer is connected with thetesting conduit through a transverse lever and transmits the measured data to the control device; the strain force transducers are arranged at different heights at intervals on the outer wall of the testing guiding pipe which is covered by seafloor soil so as to detect the stresses on the testing conduit at different seawater depths and transmit the stresses to the control device in real time; the liquid pressure gauge is used for transmitting the acquired pressure information on the upper part of liquid to the control device in real time; the output end of the hydraulic pump is connected with a water inlet on the sealed cover through a hydraulic transmission pipe; the control device is electrically connected with the control end of the hydraulic pump; and a liquid pressure threshold computing module and a friction factor computing module for the control device to control the water depth in the sealed cylinder are pre-arranged in the control device. The testing device can measure the action mechanism between the riser and the seafloor soil conveniently, quickly and accurately, and the reliability of the result is high.