Through laboratory cultivation and observation, the life history of Ulva linza which includes apomixis, asexual and sexual reproductions was found. Apomixis was completed by biflagellated gametes which were released from gametophytes. Biflagellate neutral zoospores and quadriflagellate neutral zoospores were observed. Biflagellate neutral zoospores released from zoosporangia were settled after moving for about an hour. Settled biflagellate neutral zoospores could develop into sporophytes. The newly developed sporophytes also released this kind of biflagellate neutral zoospores to complete asexual reproduction. The asexual reproduction completed by quadriflagellate neutral zoospores was the same as biflagellate neutral zoospores. Sexual reproduction was completed by an alternation of gametophytes and sporophytes. Mature dioecious gametophytes released biflagellated female and male gametes. The zygotes were formed after conjugation between female and male gametes. Then the zygotes developed into sporophytes. When sporophytes were matured, quadriflagellate zoospores were released from zoosporangia, and developed into new gametophytes. Life history of Ulva linza was typically an alternation of isomorphic, unisexual haploid gametophytes and diploid sporophytes, of which apomixis and asexual reproduction were dominant. Without conjugation between female and male gametes, the life history of Ulva linza was curtailed. The gametes could develop instantly after being settled. Moreover, the quantities of gametes released form gametangia and the quantity of zoospores from zoosporangia were very large. Under appropriate conditions, Ulva linza would reproduce rapidly. Perhaps, this is one of the reasons for the ourbreak of the “green tide” in the Yellow Sea.