The invention discloses an assimilation evapotranspiration and LAI (leaf area index) region soil moisture monitoring method, and the method comprises the specific steps: S1, calibrating a crop growth model, and completing the determination of spatial parameters; S2, generating an MODIS ET and LAI time sequence curve through superposition; S3, constructing an LAI time sequence curve through filtering; S4, calculating the one-order differential monotonicity of the curve; S5, operating an SWAP model grid by grid, outputting the ET and LAI, and calculating the one-order differential monotonicity; S6, building a cost function according to the difference between the one-order differential monotonicity obtained at step S4 and step S5, taking an seedling emergence date and irrigation time as to-be-optimized parameters, and obtaining an optimal value through employing an optimization algorithm; S7, inputting the optimal parameter into a model, and simulating soil moisture grid by grid. The method estimates the soil moisture through a data assimilation method, introduces two types of remote sensing data, improves the precision of large-area monitoring of soil moisture, and is suitable for the estimation of the soil moisture of regional-scale of farmland.