Submillimeter wave is absorbed in atmosphere seriously. So it is very difficult to detect a metal target in lower atmosphere passively. A method for detecting a metal target by a 360 GHz radiometer at high altitude is proposed. The configuration of the radiometer is presented. The contrast of the atmospheric radiation temperature to the antenna temperature of the target is calculated by using a MPM model. The radiometer is calibrated under the weather condition with the ground temperature of 0℃ and the humidity of 20% and is used to detect a 0.5 m×1 m metal target at the altitude of 3 m above the ground. The calculation and experimental results show that the model for calculating antenna temperature contrast is feasible. Under the weather condition with the ground temperature of 20℃ and the relative humidity of 20%, the 360 GHz radiometer at the altitude of 5 km can detect a 1 m×5 m metal target in the distance of 0.3 km above it and can detect a 10 m×20 m metal target in the distance of 2 km above it.