This paper reports the image pattern of spatial distribution and spectrum of optical radiation measured in the normal direction from the rear side of the target employing optical CCD camera and optical multi-channel analysor on the 100 TW femtosecond laser facility. The image pattern of spatial distribution has a ring-shape, the region of optical radiation has larger radiation angle and optical intensity distribution, and the radiation is comprised of many components. The spectrum of optical radiation shows a sharp peak near 800nm, which is attributed to the laser fundamental (ω0), and is ascribed to the coherent transition radiation (CTR) generated by microbunches of hot electron beams generated in transport. With the increase of the laser energy, the peak of the CTR moves further to the long wavelength side, the red shift is caused by the expansion of the critical density surface. The acceleration of hot electron comes mainly from resonance absorption.