The embodiment of the invention provides a rockfill density measuring method based on an additional mass process theoretical measuring plate. The rockfill density measuring method comprises the following steps: establishing a vibrating volume line graph under a wet density-vibrating mass coordinate system and a moisture content line graph based on vibrating mass, rockmill density, wet density, vibrating volume, dry density and moisture content of each measuring point on a rockmill body based on historical engineering test data; establishing a parameter peer relationship between the rockmill density and the wet density, and performing gridding coverage on the vibrating volume line graph and the moisture content line graph to obtain the additional mass process theoretical measuring plate; selecting a measuring point on a to-be-detected dam rockmill body, acquiring rockmill rigidity K and vibrating mass M of the rockmill body through the additional mass process, enabling the rockmill rigidity K and vibrating mass M to match with the corresponding vibrating volume Vo and the moisture content w on the additional mass process theoretical measuring plate, and obtaining wet density rhp anddry density rhp d of the rockmill body based on the vibrating volume V0 and the moisture content w.