The output power of the fiber laser can be increased by synthesizing the output beam, but the problem of low output power is caused by the poor ability of controlling bandwidth in the current synthesis research. Therefore, a coherent synthesis method of ultrashort pulse fiber laser based on balanced difference method is proposed. The balance difference of fiber combiner is divided into different modules, including data exchange and acquisition, circuit balancing, phase compensation, etc. Select appropriate ad conversion chip, use Ethernet chip W5300, design data exchange, ensure the normal use of data acquisition function, in the signal preprocessing module, use band-pass filter to filter out unnecessary frequency components, use AD603 to design front-end balancing circuit, design signal separation circuit, separate AC and DC signals, use active phase lock. The phase compensation of each laser is realized by the fixed technology, and the coherent synthesis of ultrashort pulse fiber laser is realized by the fiber combiner. The experimental results show that, compared with the traditional coherent synthesis method, the output power of the coherent combination method based on balanced difference method is about 900 kW, which has higher output power and better control bandwidth. It provides a theoretical basis for practical application.