This paper considers the K dimentional dyadic expansion of composite pseudorandom code waveform function in Galois (qⁿ) finite field by the use of number transform，and bui1ds up a simple and available mathematical model. The expansion method，basis function and properties of sequency spectrum have been analyzed. As an example for its application，this paper provides the method of analyzing problems of composite pseudorandom codes with the use of this expansion. These problems contain the correlation function，the power spectrum，the spread spectrum and the dual matched filtering with progressing step by step.