Based on the principles of planar near field acoustical holography (PNAH) in the semi free field, the results computed by numerical methods are used as holographic data to reconstruct an image of underwater noise radiated by a ship. The relationship between reconstructive results obtained by propeller noise and flow noise and reconstructive distance is investigated. The results show that, the underwater noise radiated by a ship can be identified by PNAH in the semi free field. It is so sensitive to changeable reconstructive distance using PNAH in the semi free field that the applicable range of reconstructive distance is limited. Propeller noise is the dominating noise near the stern of the ship.