The effects of precipitated phase on the microstructure, deformation mechanism and edge crack behavior of AZ91 hot rolled magnesium alloy plate with different reduction per pass and rolling passes were studied by optical microscope, field emission scanning electron microscope equipped with energy spectrum and tensile testing at ambient temperature. The results indicate that multi-pass with small rolling reduction can reduce the edge cracks and improve the rolling forming ability for AZ91 magnesium alloy; Precipitated phase from the slice layer to spherical particles with tinier size during the rolling deformation is beneficial to inhibit the initiation of crack; The tensile fracture surfaces show obvious intergranular fracture features, and the microcrack primarily distributes around the Mg/Mg₁₇Al₁₂ phase interface and coarser second phase. Spherical precipitated phase particles have been proved to be the initiation source of microcrack, and then the microcrack extends and turns to the microcrack.