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To improve the performances of separate absorption and multiplication AlGaN avalanche photodiodes (APDs), a polarization field and a polarization doping effect are introduced into the APDs by adjusting the Al composition of the p-AlGaN layer. The calculated results show that the polarization-induced electric field, which has the same direction as the reverse bias in the multiplication layer, can significantly lower the avalanche breakdown voltage. Further, the maximum multiplication gain increases pronouncedly due to the polarization-assisted enhancement of the ionization electric field at the point of maximum gain. In addition, the composition graded p-AlGaN layer can also improve device performances through the polarization doping effect. The employments of the polarization field and polarization doping can increase the maximum multiplication gain by as much as 225%.