It is difficult to accurately detect the boulder in weathered granite ground. As a result, shield machine boring in weathered granite ground is of great risk. The boulder detection method of ground penetrating radar combined with drillhole is studied by theoretical analysis, numerical simulation and site test. The study results show that: 1) The boulder diameter larger than 0.7 m and within 3-5 m scope around drillhole can be detected by setting 100-200 MHz radar antenna. 2) The reflected waves of boulders of different sizes in uneven strata vary greatly; and the size and cover depth of boulder can be accurately detected by observing the strength and velocity of reflected wave. 3) The site test results of weathered granite ground coincide with the actual results, which proves the rationality of the detection method. The boulder detection by ground penetrating radar combined with drillhole is worth popularizing.