The present invention relates to the technical field of rotary steerable drilling, particularly to an apparatus for testing the function and performance of a rotary steerable tool in a horizontal or inclined state. The apparatus comprises a rotary steerable tool testing bench configured to provide a horizontal state and an inclined state for a rotary steerable tool so as to simulate a horizontal drilling state or a directional drilling state on the ground; and a rotary steerable tool function and performance testing unit disposed on the rotary steerable tool testing bench to test the function and performance of the rotary steerable tool in the horizontal drilling state or the directional drilling state. The apparatus in the present invention can test the function and performance of a push-the-bit rotary steerable tool under horizontal and inclined conditions on the ground in a lab.