Neutron porosity measurement could provide basic and important parameters of the formation while drilling and it is indispensable for the real-time comprehensive formation evaluation and geosteering. As deeper and more complex reservoirs are being drilled, the needs of slim-diameter tools are increasing. This paper presents a new neutron porosity LWD tool using a generator source. The tool can operate in environments with pressures up to 172 MPa and temperatures to 175 degC, and can be combined with the 4.75-in geologic steering drilling system.Most neutron porosity tools, including wireline and LWD devices, use Americium-Beryllium (AmBe) and Cf-252 radiological sources which have great impact on the environment and have problems of handling, shipment and storage. This tool in hardware includes a pulsed neutron generator (PNG) and three Helium-3 (He-3) filled detectors. With two different source distance combinations, the tool could be working in multi-detection or high-sensitivity measurement mode which would adapt the tool to different kinds of formation. Standard well tests show that the data of this tool is in good agreement with the wireline logging tools. It also get good results in temperature and pressure test. All these preliminary tests and data analysis show that the tool could be very helpful in formation evaluation and reservoir monitoring in HT-HP conditions. This paper also presents compatibility design of the tool and possible applications as a part of the 4.75-in geosteering drilling system.