The invention provides a lake ice freezing and thawing monitoring method and system based on a dynamic mixed pixel decomposing method. The method comprises the steps: S1, obtaining a lake mixed pixel brightness temperature value and a lakeshore pure pixel brightness temperature value based on a lake specific optical image and passive microwave remote sensing data; S2, obtaining area proportion of lake to lakeshore in a lake mixed pixel through overlay analysis based on the lake specific optical image and the lake mixed pixel brightness temperature value; S3, utilizing a linear mixed pixel decomposing method to obtain lake pure pixel brightness temperature value of the lake mixed pixel based on the lake mixed pixel brightness temperature value, the lakeshore pure pixel brightness temperature value and the area proportion of the lake to the lakeshore in the lake mixed pixel to monitor the lake ice freezing and thawing phenomenon. The method disclosed by the invention enables the passive microwave remote sensing data to be applied to monitoring freezing and thawing of sub-pixel lakes in high Asian areas to obtain lake brightness temperature information in sub-pixel and achieves high Asian area passive microwave remote sensing sub-pixel lake ice freezing and thawing monitoring.