Abstract: Taking mullet living in Qinshan area as an example, main tissues and organs were measured by sampling on site, and an anatomic model of mullet was developed based on the biological tissue structure and activity concentration distribution in tissues and organs. The energy absorbed fractions of particles emitted by ¹³⁷Cs and ⁹⁰Sr in different tissues and organs were simulated by MCNP code, and then the result of simulation was used to calculate radiation doses to mullet. At the same time, a homogeneous unitary model with the same geometry was developed to estimate the dose to mullet in a traditional way for comparison. The conclusion is that for internal exposure, in the instance that some internal organs have high concentration factors, the homogeneous unitary model for estimation of internal exposure to mullet is not conservative, because parts of the organism will get more doses, but for the external exposure, the result calculated by anatomic model is smaller.