The invention discloses a
projection lens system. The
projection lens system sequentially comprises a first lens with positive focal power, a second lens with negative focal power, a third lens with positive focal power and a diaphragm from an object side to an image side along an
optical axis, and is characterized in that an object-side surface of the first lens is a convex surface; an image-side surface of the third lens is a convex surface; and the optical centers of the lenses are located at the same straight line. The
projection lens system satisfies the following conditions: f12>f3, (dn / dt)1<-50*10<-6> / DEG C, (dn / dt)2<-50*10<-6> / DEG C, and (dn / dt)3>-10*10<-6> / DEG C, wherein f12 represents the combined
focal length of the first lens and the second lens, f3 represents the
focal length of the third lens, and (dn / dt)1, (dn / dt)2 and (dn / dt)3 respectively represent changes rates of refractive indexes of the first lens, the second lens and the third lens along with the temperature. The projection lens system can cancel out influences imposed on the
focal length by
thermal expansion brought about by the lenses and structural members due to reasonable distribution of the change rate of the
refractive index of each lens along with the temperature, thereby being capable of realizing stable focal length and being applicable to different temperature occasions.