Abstract: The calculation method of ambient absorbed dose rate for a mobile large container inspection system developed by Tsinghua Tongfang Limited Company is presented using the general code MCNP. Under various loading, ambient absorbed dose rates along the fence of 30 m×30 m and 40 m×40 m, at 1.5 m high above the ground are calculated. The ambient absorbed dose rates distribution without loading is measured experimentally, and a comparison between calculated and measured results is carried out. It is shown that both results are in agreement within 40 %.