With horse mackerel as experimental material, the temperature distribution and migration characteristic of moisture during fish fillets drying by heat pump dehumidifier (HPD) were studied. The results showed that temperature distribution inside horse mackerel fillets is uniform during the great part of drying periods, and the regular temperature gradient along the direction of fish fillets thickness doesn’t occur. During horse mackerel drying by HPD, an obvious moisture gradient along the direction of thickness of fillets is found with drying temperature and velocity. In the initial stage of drying, drying rate of fish fillets is mainly depended on moisture evaporation. In the next stage of drying, drying rate of fish fillets is mainly depended on moisture diffusion. The moisture migration is mainly controlled by moisture content gradient.