The invention discloses a drilling fluid damage calculation method for a horizontal well. The method comprises the following steps of obtaining the total length L of a horizontal section of the horizontal well, dividing the horizontal section into m sections with the length of delta l, and dividing each section into n small sections; initializing parameters, and for each small section, applying the initial conditions: rc = rw, q = qsl; updating the shaft radius rc (i) and the drilling fluid filtration flow q (i) for each small section; solving the accumulated filter loss Q and the damage radius rd until the soaking time is over; converting rd into rdH in a horizontal direction, and solving the permeability Kd of each small section in the horizontal direction after equivalent damage; and according to an epidermal coefficient equation, solving the epidermal coefficient of each small section until each section of the length delta l is calculated, and obtaining the epidermal coefficient distribution along the horizontal section. The method is used for solving the problem that in the prior art, the horizontal well skin coefficient obtaining difficulty is large, and the purpose of obtaining the horizontal well skin coefficient distribution through rock core data is achieved.