The method of the application provides a determination method and a device for a branch horizontal well forecast profile, wherein the method comprises the steps that three-dimensional seismic data of a target area and wellbore data of a plurality of drilled wells in the target area are obtained; according to the wellbore data of the drilled wells and the three-dimensional seismic data, connected graphs of a plurality of branch horizontal sections are established on the two sides of a main horizontal section by a virtual well; and according to the connected graphs of multiple branch horizontal sections, the branch horizontal well forecast profile is determined. The scheme uses the data of the drilled wells and the three-dimensional seism to establish the virtual well, the space information (such as well depth and oil layer thickness) of the virtual well is obtained, then vertical connected graphs of the branch horizontal sections are established, and the three-dimensional branch horizontal well forecast profile is established, so that the technical problems that an existing branch horizontal forecast profile method can only construct and use a two-dimensional profile and can not represent the oil layer space information, so that prediction accuracy is poor, data representation is inaccurate are solved.