  We present an improved inverse ray-shooting code based on GPUs for generating
microlensing magnification maps. In addition to introducing GPUs for
acceleration, we put the efforts in two aspects: (i) A standard circular lens
plane is replaced by a rectangular one to reduce the number of unnecessary
lenses as a result of an extremely prolate rectangular image plane. (ii)
Interpolation method is applied in our implementation which has achieved an
significant acceleration when dealing with large number of lenses and light
rays required by high resolution maps. With these applications, we have greatly
reduced the running time while maintaining high accuracy: the speed has been
increased by about 100 times compared with ordinary GPU based IRS code and
GPU-D code when handling large number of lenses. If encountered the high
resolution situation up to $10000^2$ pixels, resulting in almost $10^{11}$
light rays, the running time can also be reduced by two orders of magnitude.
