The characteristics of terahertz (THz) radiation from the surface of an (100)p-InAs crystal excited by fs laser pulses of different wavelengths (from 750 to 850nm) are investigated experimentally. The terahertz radiation can be interpreted as being emited from accelerated photo-carriers in the Dember field and be detected by using the free-space electro-optic sampling method. A (110) ZnTe crystal was used as detector. The results of experiment show that the Dember field in the surface of InAs, concentration of excited carriers, intervalley scattering and the concentration of excited carriers of different states, all changed when the wavelength of excited pulse changed, so the radiant efficiency and the effective spectral width of terahertz wave were different. This investigation will be useful to the measure of time domain spectrum of samples and the optimization of experimental system. It also gives better physieal insight into the ultra-fast process of terahertz wave radiation from InAs excited by femtosecond pulses.