In this paper, a new equidistant offset filling theory based on level set function and corresponding numerical solving algorithm based on dynamic finite difference method were proposed. Firstly, the closed curve is defined as the zero-value contour of level set function. Then, the level set equation, a partial differential equation, has been built to get the level set function, and a dynamic finite difference method has been proposed to solve the level set function. Secondly, three types of cross-sections which are simple polygon, multi-island polygon and multi-hole polygon were used to test the equidistant offset filling effect of the algorithm. The test results show that the proposed theory and algorithm in this paper can well deal with the equidistant filling problem of these cross-sections. In addition, any complex section can be equidistant offset filled by the algorithm proposed in this paper, and the calculation cost is significantly related to the number of offsets, but not directly related to the section complexity. Finally, the remanufacturing of a typical crankshaft hot forging die proves that the algorithm proposed in this paper can effectively complete the remanufacturing repair of this kind of forging die. The surface of the die is smooth, and there are no defects such as porosity and slag inclusion after machining. Compared with the new die, the manufacturing cost can be saved more than 50%, and the efficiency of die manufacturing can be improved more than 60%. This is due to the machining allowance of the die after additive manufacturing is very small, and it can be quickly repaired on site.