A fastening device for projector lamp. The fastening device includes a lamp, a lamp casing and a strip-shaped spring that fastens the lamp onto the lamp casing. The lamp casing has a first sidewall, a second sidewall, a bottom wall and an upper connective structure, together they encloses a central space. The first sidewall has a first opening and the second sidewall has a hook groove. There is a second opening between the second sidewall and the bottom wall. The spring has a first end and a second end. A central portion of the strip-shaped spring can be latched by the hook groove. The first end of the spring is fixed by the first opening on the first sidewall. The second end of the spring has an internal recess structure. The second end of the spring passes through the second opening and fastens onto a recess groove on the lamp. The deform stress produced by passing the spring through the first opening, the hook groove and the recess groove is capable fixing the lamp relative to the lamp casing.