The flow measurement sensor based on small diameter electromagnetic correlation method has a broad application prospect in the field of petroleum production logging, and the design of the axial length of the excitation coil is one of the key technologies to made the sensor. By analyzing the magnetic theoretic model of the sensor’s excitation coil in measuring area and electromagnetic flowmeter measurement principle, a new magnetic field analysis index is proposed, and numerical simulation with different velocity and different excitation coil axial length is carried out. The axial length of excitation coil is analyzed and determined by comparative analysis new magnetic field analysis index and traditional magnetic field evaluation indexes. The experimental platform is established to carry out preliminary experimental verification, which provides a solution for the optimization design of this kind of sensor excitation coil. At the same time, it speeds up the development process of flow measuring instruments based on electromagnetic correlation method.