The invention provides a direct radiation prediction method based on a conventional weather forecast. According to a weather type classification algorithm, a preferred statistical method is adopted to classify weather types, and a weather type classification model is obtained; according to the weather type classification model and historical direct radiation data, a weather type curve model is built; and according to the weather type curve model and inputted conventional weather forecast data, a direct radiation prediction model is built, and direct radiation prediction data for future 24 hours are obtained. The method provided by the invention overcomes influences on direct radiation prediction by season changes, solves the problem of missing of the direct radiation data to acquire more accurate direct radiation data, and accurately predicts the direct radiation of a photothermal power plant in the future 24 hours; the calculation precision is high, the needed calculation resources are few, good prediction effects can be achieved, and a data basis is laid for power generation power prediction for the photothermal power plant; and further, normal operation of the photothermal power plant can be ensured.