A device for testing a needle roller bearing of a planet gear set consists of two identical helical planet gear sets, a piston, an end cover, a spindle, and can simulate the actual operating condition of a needle roller bearing of the planet gear set; the rotating speed difference between an inner raceway and an outer raceway of the needle roller bearing is determined by adjusting the rotating speed of a sun gear and a planet carrier; the load on the needle roller bearing includes a centrifugal load and a radial load, and the centrifugal acceleration and the centrifugal load are determined by adjusting the rotating speed of the planet carrier; a method of determining the radial load by adjusting the hydraulic pressure difference of hydraulic chambers at both ends of the piston, generating a tangential force to load the planet gear, and balancing the radial load of the bearing.