Based on the scalar diffraction theory, the intensity distribution of accelerating quad Airy beams(AQABs) at different positions is numerically simulated.On the basis of it, through the research of the Poynting vector of AQABs, the local energy flow of electromagnetic fields of AQABs is described and it also provides intrinsic mechanism as to how AQABs generate and accelerate in free space. Further study of its angular momentum shows that AQABs are not a simple superposition of four classical Airy beams but an organical entity whose four parts of Airy beams have completely different optical properties. The research results reveal the intrinsic optical nature and transmission characteristics of AQABs.