Necessary critical curvature values of structural fracture formation was determined and calculation model of constructional curvature method was set up based on principal curvature formula of volcanic target stratum structural surface in research area through differential calculus principle, along with principle and prerequisite for application of constructional curvature method in volcanic reservoirs structural fracture prediction. Taking gas reservoir of volcanic target stratum in Anda Xujiaweizi area as example, fracture development of target stratum was predicted by constructional curvature method, which showed that high value area of structural main curvature and log fractal dimension coincided quite well with high output gas well part.