A crystallizer and method for casting using the crystallizer. The crystallizer mainly includes a plurality of-position-limiting parts (16) on the inner side of the mould seat (6, 7). The inner side of the position-limiting parts (16) is in correspondence with the external periphery of the mould wall (8-1, 9-1) of the film mould (8, 9). A medium channel (17) with a medium-supplying port (5) at its upper end is formed between the adjacent position-limiting parts (16).