The invention provides a field operation method for precisely determining the value of the shear strength of a rock mass structural plane of a mine slope. The method comprises the following steps: investigating engineering geological conditions in the range of slope rock in a field surface mine, dividing different engineering geological zones, and then dividing the mine slope into different areas according to obvious changes of engineering parameters of the slope; measuring the height and width of each area of the slope and measuring slope aspect and slope angle; carrying out geologic description according to the divided areas of the slope, separately measuring the occurrence and length of every structural plane and distance between structural plane groups, and describing the filling situation of the structural planes; subjecting the occurrence of the slope and the occurrence of the structural planes to stereographic projection, analyzing possible failure modes of the mine slope, and determining a potential slip plane of the mine slope and a slipping direction thereof; finding out a structural plane corresponding to the potential slip plane and measuring the rebound value of wall rock of the structural plane; and drawing the surface profile curve of the structural plane along the potential slipping direction on the structural plane corresponding to the potential slip plane. The field operation method provided by the invention is simple in principles, convenient to operate, low in cost and good in usage effect.