To study the photochemical processes of the azide in solid polymer matrix, poly(styrene-co-vinyl benzyl azide) (AZMS) was synthesized. Sensitized photodecomposition of the azido group was investigated for the solid film of AZMS/sensitizer systems. The amount of the evolved nitrogen under a steady-state condition was measured, and the energy transfer and decay rates of the triplet sensitizer were determined by use of a Nd:YAG laser flash photolysis technique. From analysis of the experimental results, the azido group in solid AZMS film was found to decompose efficiently by autocatalytic chain decomposition. Furthermore, the external magnetic field effect on photocrosslinking reaction was observed with thin AZMS polymer film. Application of a magnetic field of 0.089 T decreases the photocrosslinking efficiency by ca. 20%. Based on the photocrosslinking mechanism, chemical behavior of the singlet nitrene in solid AZMS film was discussed.