The present disclosure provides a secondary battery and a top cover assembly thereof. The top cover assembly includes: a top cover plate and a valve sleeve. The top cover plate includes an accommodation groove and a valve part protruding upward from a bottom wall of the accommodation groove, and the valve part is provided with at least one gas exhaust hole that communicates with an interior of the secondary battery. The valve sleeve is located in the accommodation groove and covers the valve part, a gas exhaust passage that communicates with outside is disposed between the valve sleeve and the valve part, and the gas exhaust passage communicates with the at least one gas exhaust hole.