The dielectric constant of rock is an important parameter to characterize its electromagnetic radiation characteristics, which is of great significance to remote sensing observation of the Earth. However, the existing methods for measuring dielectric constant usually have specific requirements on the shape and size of samples, and the measured dielectric constant can only represent the dielectric characteristics of local small areas of rocks, but cannot characterize the dielectric characteristics of rocks as a whole. Therefore, based on the theory of microwave interference, a new method for measuring the dielectric constant of rock by microwave radiometer is studied and proposed in this paper. Then, based on this method, the dielectric constants of granite and sandstone are measured by C-band microwave radiometer. In order to verify the correctness of the test results, the AET open coaxial resonator method is used to compare and verify the results. The results show that the dielectric constants of granite and sandstone measured by the new method are 3.89 and 5.71 respectively, and the relative errors are 3.2% and 2.7%, respectively, compared with those measured by AET open coaxial resonant cavity method. The research results show the feasibility of the new method, which is a useful supplement to the current point dielectric constant measurement method.