The morphological development of the juvenile of Hyriopsis cumingii was observed and described from the glochidia that fell off the host fish to the triangle in a hatchery at the water temperature of 26.5-32.0 ℃. According to the morphological characteristics, the juvenile development of Hyriopsis cumingii could be divided into four stages. The average shell length of newly fallen juvenile was 221.88 μm, and the sail had not been formed, so its total height was equal to shell height. The width of the shell was expanding quickly in the first four days after falling off, named as width expanding stage. From fifth to twentythird days, the juvenile developed into umbo extrudeing stage, because the umbo began to extrude. There are apparent changes during this stage that the posterior shell was much longer than the anterior shell after fifteen days. On twentythird day, the wing appeared, so the juvenile began to develope into wing forming stage. From thirtyfourth to sixtieth day, the sail gradually grew and the ratio of total height to shell height increased. The umbo was no longer the top of the juvenile and it developed into sail growth stage. On sixtieth day, the development of the juvenile stage of Hyriopsis cumingii had completed and entered the young stage, with similar morphological characters to those of adult. The position of the umbo represented the most apparent changes during the juvenile development of Hyriopsis cumingii. The relationship between total length and day age could be expressed as 〖L=329.39e^0.059t（r=0.968）, that between total height and shell length as 〖H=0.776L-103.36（r=0.997）.