The vortex beam is a kind of spatially structured optical beam carrying orbital angular momentum, whose frequency shifts when it illuminates the surface of a rotating object. This phenomenon, known as the optical rotational Doppler effect (RDE), can be used to obtain the rotation frequency of a flat object by measuring the frequency shift. While the frequency shift is influenced by the incident condition, by revealing the influencing law of incident condition, the rotational frequency of the object can be measured directly. Firstly, a method of velocity projection was used to analyze the mechanism of optical RDE. Then, the rotational Doppler frequency shift distribution law at general incidence of vortex beam was obtained, and the theoretical method of extracting the rotational frequency was proposed. In the end, an experiment of RDE using a superimposed Laguerre-Gaussian beam with topological charge $l = \pm 18$ was set up, and rotational Doppler frequency shift spectrum at 4 incident conditions was obtained. The experimental spectrum and the theoretical result were combined, then rotational frequency of the object could be extracted with an error less than 1%.