Abstract: To quantitatively estimate the effect of returning cropland to enclosure on non-point source pollution in Liaohe Conservation Area, the dynamic change of cropland from 2010 to 2018 was analyzed by using high-resolution remote sensing data, and the inflow of pollutants from cropland area surface runoff was estimated by employing the method of source intensity coefficient. The results showed that lots of croplands distributed on both sides of the river in 2010, with an area of about 627.13 km², accounting for 33.55% of the total area. The COD and NH₃-N inflows from cropland surface runoff were 1 864.79 and 372.96 t/a in 2010, respectively. With the measure of returning cropland to enclosure being taken, the cropland area decreased to 155.63 km² in 2011, with the rate about 75.18%, and only scattered on both sides of the river. The COD and NH₃-N inflows from cropland surface runoff decreased to 419.55 and 83.91 t/a, respectively. From 2012 to 2018, the cropland area and the inflow of surface runoff pollutants were generally consistent with that in 2011. Compared with that in 2010, the measures of returning cropland to enclosure reduced the COD and NH₃-N inflows from farmland surface runoff by 11 734.49 and 2 346.90 t, respectively, from 2011 to 2018.