A theoretical study on the polyamic acid (PAA) and its derivatives with different side chain (GPAA1-4) by semi-empirical PM3 is presented in this paper. After optimization, the authors get their balanceable geometric configurations. For all optimal geometric configurations, there is no imaginary frequency in vibrational analysis, which proves that the geometric configurations are stable. On this basis, the electronic spectra have been calculated by a single-excitation configuration interaction (CIS) method. Tie calculated results are consistent with experimental values on the whole. The authors found that the fluorescence intensity of polyamic acid (PAA) with p-pi conjugation side chain is enhanced obviously, and the longer the alkyl side chain the more intense the fluorescence will be. This will help us to design and synthesize the polymer fluorescence material.