The space charge in air is closely related to the mechanism of corona discharge. In order to study the onset and sustainability of corona discharge, acquiring the distribution of space charge is necessary but there still exists a puzzle which has not been settled. According to the sound pulse method, in this paper we present a kind of signal processing algorithm to analyze the electric field which is generated by modulating the space charge in the sound field. The electric filed is dependent on the form of sound emission and space charge density. The waveform of electric field is related to space charge density. Through the proposed algorithm, the space charge density can be obtained by analyzing electric field signal. The area in which the space charges need to be measured, is divided into elements. Each element is small enough so that the space charge quantity in each element is assumed to be the same. The following assumption is accepted during numerical simulation: space charge densities in the wave fronts are the same. The curve of electric field produced, received by electric field antenna, is the vector sum of electric filed produced by each element, and then calculated by numerical simulation. In order to satisfy the assumption in each measurement case, the requirements for sound emission system under different cases are discussed. In different cases, different sound emission systems are required. For space charges which are distributed uniformly, plane wave or spherical wave is suitable; for one-dimensional space charge distribution, plane wave is necessary; for space charge two-dimensional or three-dimensional space charge distribution, plane wave array is availed. What is more, a corresponding measuring system is developed which can be used for measuring the space charge density. This system mainly contains the producing of sound pulse, producing of space charges and the receiving of electric field signal. The producing of sound pulse is designed according to the measurement requirement for multi-needle-to-plate geometry which is assumed that space charge is distributed uniformly in the gap. With the experimental model, the space charge density in multi-needle-to-plate geometry is calculated according to the algorithm proposed in this paper. The result is compared with the calculated one by the method of corona currents, verifying the proposed method.