The present invention discloses a dynamic strain field measuring method and system for a rotor blade based on blade tip timing. The method includes the following steps: establishing a three-dimensional finite element model of a to-be-measured rotor blade, and extracting modal parameters of the three-dimensional finite element model; determining the number and circumferential mounting positions of blade tip timing sensors; establishing a mapping relationship between single-point displacement and full-field dynamic strains of the blade; acquiring blade tip single-point displacement of the rotor blade based on the blade tip timing sensors; and realizing, by the single-point displacement, dynamic strain measurement in any position and direction of the rotor blade based on the mapping relationship.