In order to distinguish grey model or grey Markov model is more suitable for rainfall forecasting, grey model(GM), unbiased grey model and moving unbiased grey model were compared by taking the Caizhuang Reservoir in Shouyang County, Shanxi Province as the research object and using its 40 a rainfall data. The model with the highest precision slinding unbiased grey model was chosen and added to Markov model to generate grey Markov model and then compare it with grey model. Results showed that grey model gave a poor result for long-term prediction and highly fluctuant data. Therefore, based on grey model and Markov model, improved grey Markov model was established, which had their respective advantages and better predicting effects.