The growth of Mg doped AlGaN films grown by metal organic chemical vapor deposition is investigated. Through studying the effect of high temperature AlN interlayer, high temperature AlN can effectively adjust the strain of the films and significantly improve the quality of films, reducing the density of dislocation. With the increase of Al composition, a large number of islandshaped crystal nuclei arise and the quality of films dramatically decreases. Threedimensional growth mode is difficult to be transformed into twodimensional one. Both the increase of the fraction of Al and Cp2Mg flux can increase the density of screw dislocations. The increase of Cp2Mg flux can dramatically increase the density of edge dislocation, while the fraction of Al has little impact on the density of edge dislocation. Finally, the influence of annealing temperature on the hole concentration of Mg doped Al0.1Ga0.9N sample is studied, finding that 900℃ is the ideal temperature to obtain the Ptyped AlGaN.