Abstract: In order to manipulate electromagnetic wave in both space and frequency domain using metasurface, this paper presents an analytical method which can solve space-time modulation design. Firstly, the ideal time-varying scattering coefficient is introduced according to the requirements of manipulation. Then, a series of rectangular pulses are used to discretize the ideal coefficient, and the unit-cell with specific responses are designed accordingly; Fourier series is implemented to highlight the contribution of the different frequency harmonics. The state of response, start time, and width of each pulse are optimized analytically, to only keep the desired harmonic and suppress the unwanted ones. The agreement of analytical and full-wave simulation results provides strong support for the validation and accuracy of the proposed method. The mechanism of electromagnetic wave control, in space/frequency domain, based on space-time modulation is studied intensively in this method, which may pave a new way for the applications of space-time modulated metasurface in radar, communication and other fields.