This paper tries to build a multi-functional downhole nuclear magnetic resonance (NMR) fluid analysis laboratory that can evaluate fluid information in real time at reservoir conditions at a depth of several thousand meters. The aim is to monitor the pollution of the formation fluids and quantitatively evaluate NMR characteristics of the fluids. It focuses on the design of the structure and parameters of a sensor with zero stray fields. Two separate coils are designed to measure NMR characteristics of flowing or static fluids. A method is proposed to use the Bloch equation, to guide the optimization of the NMR sensor. Finally, the measured results confirm that the design is reasonable. There is a homogeneous static field (perpendicular to the axial direction) in the center of the sensor, and there are no stray external fields. The novel design of pre-polarization magnet improves the signal to noise ratio, while shortening the sensor length.