The invention relates to a method for determining a safe mud density window of a special stratum. The method comprises the following steps of 1) determining rock mechanics parameters of a sedimentary rock stratum according to an acoustic time difference, density and natural gamma data obtained by target well logging and an indoor rock mechanics experiment; 2) according to natural gamma logging data and an actual measurement value of stratum bearing capacity at a drilling site, determining geomechanics parameters; 3) calculating a surrounding-well stress state by utilizing the geomechanics parameters; 4) judging whether the stratum has a collapse pressure upper limit or not; and 5) according to a symbolic-graphic combination principle, for three deformation and fracture forms, determining a collapse pressure lower limit, a fracture pressure and the collapse pressure upper limit, and obtaining the safe mud density window of the special stratum. By utilizing the method, the special stratum with the collapse pressure upper limit can be effectively and accurately identified, and the safe mud density window of the special stratum is determined.