It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such a kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses is formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from modulated asymmetric laser fields. It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from the modulated asymmetric laser fields.