A pneumatic saw includes a housing, a piston casing, a piston, and an air guide assembly. The air guide assembly is in the housing, and includes a reversing brake block, an air guide base, a spacer plate on the air guide base, and an air blocking ring on the air guide base. The air guide base includes an air inlet duct for accommodating the reversing brake block, and primary working air ducts communicated with the air inlet duct, and secondary working air ducts communicated with the air inlet duct. The primary and secondary working air ducts respectively includes a longitudinal hole parallel to the air inlet duct, and a transverse through hole communicated with the air inlet duct and the longitudinal hole. The transverse through hole is shielded by the air blocking ring, and the air blocking ring is fixed to the air guide base by a pin member.