A torsion spring includes a spring body wound by a helical wire, a first arm extending tangently from an end of the spring body, and a second arm extending from the other end of the spring body. The first arm includes sequentially a first portion extending from the spring body, a curved second portion bent toward a first direction from the first portion, and a curved third portion bent toward a different second direction from the second portion. A torsion spring assembly and a releasable casing using the torsion spring are also disclosed.