Using similarity principle and Chebashev Polynomial, the altitude field for three layers at high altitude in the earlier stage of rainstorm was expanded and the field-similarity characteristics of altitude field came into prominence.The Q vector divergence, water vapour flux and flux divergence, potential unstability and other physical quantities closely related to rainstorm were analysed. Also, by futher approximating the main characteristics in field distribution, the optimum analogue forecast and analogue analasis forcast were given.