A differential accelerometer comprising of two rotating masses made of the same material is proposed for drop tower-based free-fall testing of the spin-spin force between the rotating mass and the Earth.The measurement is performed by placing the two concentric masses of very different momenta in a vacuum drop capsule which is falling freely in the Earth’s gravitational field.A nonzero output of the differential accelerometer is an indication of possible violation of new equivalence principle (NEP).We present the conceptual design of a modified free-fall NEP experiment which can be performed at the Beijing drop tower.Design and evaluation of the differential accelerometer with a hybrid electrostatic/magnetic suspension system are presented to accommodate for operation on ground and drop-tower tests.Details specific to the measurement uncertainty axe discussed to yield an NEP test accuracy of 7.2 × 10-9.