Abstract: In order to analyze the type and characteristics of downhole formation fluid in situ under LWD conditions, an analysis method for downhole NMR fluids while drilling has been advanced under the constraining condition of LWD applications, so as to simulate the physical space at the constraint of drill collar size and optimize the design of small NMR analyzer. On this basis, a small NMR fluid sensor suitable for the space condition of downhole drill collar was developed. Then the team created a full-size prototype of a downhole fluid-while drilling NMR analyzer. The T₂ spectrum experiment results of downhole fluid samples showed that different fluid signals had obvious response characteristics and high signal-to-noise ratio. Downhole NMR fluids analyzer while drilling can provide a new and effective test method for downhole fluid analysis, and they lay a foundation for the application of NMR technology in downhole LWD.