Abstract: A new 32-electrode data acquisition system for multi-frequency EIT has been designed. In this system, sweeping frequency combined with mixing frequency technique is employed. With its four working modes and control programs, it can collect impedance information at four different frequencies simultaneously from 1.6kHz to 380kHz in order to demodulate the signal with more than one frequency in a single set of measurement, a new digital demodulation method. which has the advantages of high calculation speed and high precision, has been developed and named by us as 'orthogonal sequential digital demodulation'. For a signal with 'n' components of frequencies and sampling N points per period, using this method. the total needed operations is 2nN multiplication and 2n(N-1) addition operations, and the signal-to-noise ratio can be improved by (N/2)0.5times.