Abstract: The short-distance free-space transmission characteristics of the optical orbital angular momentum (OAM) beam were experimentally studied. The transmission distance is 0~50 m indoors. A digital micromirror device (DMD) was used in the experimental setup to generate the OAM beam. At the receiver, a spatial beam analyzer was used to measure the intensity pattern of the OAM beam. The beam broadening effect of the OAM beam at different transmission distances was studied. The phase pattern of the OAM beam was studied by the interferometric method. At the receiver, a single path Sagnac interferometer (SPSI) was used to separate and detect the intensity of modes of the OAM beam. The effects of energy migration from the sending mode to the sideband modes of the OAM beam were studied. A heater was used to generate the strong turbulence to simulate the influence to the OAM beam mode transmission characteristics. The experimental results show that the OAM beam with large mode topological charge has more deterioration of the mode purity after transmission in the strong turbulence.