In order to understand the strength property of platy rock mass, in-situ tests for the strength of the structure plane in dolomite platy rock mass at Xiaoyouying in Fangshan District of Beijing are carried out. Traditional in-situ tests will result in a high cost, and bring more difficulties for the shear strength of rock mass. To get rid of the heavy reaction frame, according to the relationship between the attitude of platy planes and the direction of tectonic stress in the rock mass, three test rock blocks, laterally disconnected from their parent rock in different forms, are dug out vertically on the horizontal ground of the test site. By means of the test method explained above, and based on the data of the local tectonic stress in Beijing territory, a pretty good result has resulted from the in-situ test for the shear strength (c, φ) of platy planes in the dolomite platy rock mass at Xiaoyouying in Fangshan district of Beijing.