The study monitored phosphorus levels in rainfall runoff from a small watershed in Yimeng mountainous area. Results showed that the first rainfall had higher phosphorus concentrations than the maximum intensity rainfall. Phosphorus output in the first rainfall was also higher, except for dissolved organic phosphorus. The two special rainfalls contributed significantly to the total phosphorus output. The first flush effect was observed, with the maximum intensity rainfall having a stronger impact. Runoff was found to be linearly related to nutrient output, with differences in slopes between the two rainfall events. To reduce agricultural non-point source pollution, it is suggested to limit practices like tillage and fertilization during special rainfall periods and implement measures like building canals and ponds.