In this paper, modulated metasurface based multibeam antenna is presented. The impedance superposition method of all impedance modulations for different beams is utilized in the design. The current reported multi-beam modulated metasurface has a very small number of beams due to the mutual interference of different impedance modulations. It is found that the source locations for each beam play a key role in interference suppression. Instead of using the time consuming full-wave simulation-based optimization, in this paper, the optimal source locations are obtained by using the aperture fields calculated from the zeroth-order approximation of the currents on the metasurface. A five-beam modulated metasurface antenna has been designed, providing an angular coverage up to ±30°. The peak realized gain for the broadside beam is 21.3 dBi at 14.30 GHz.