Social work in the United States has a history of hundreds of years, and the values, methods and techniques of social work are used in professional places, which are called occupational social work. The union is an important social organization in the professional place. In the course of history, the relationship between trade union and social work is different in different times. Union and early social work’s cooperation and contradiction coexist.After a brief period of cooperation between the two during world war ii, the relationship was lost. After the 1960s, union and social work became more closely united.The union began to introduce social work voluntarily, and social work began to reflect on the disadvantages of being away from the union. Through the review of American historical experience, it is believed that the development of Chinese social work, especially occupational social work, must pay attention to the power of the union. Under the new situation, China's trade unions should strengthen reform, change the union focusing on rights protection and service. Social work is a resource that can be introduced.The education and research of social work should not ignore the role of trade union.