In order to accurately determine the effective pore size of needle-punched nonwoven geotextiles, the wet sieving method and the low-field nuclear magnetic resonance (NMR) technology were used to carry out experimental research. The relationship curves between the cumulative percentage and the pore size were drawn, and the effective pore size value of nonwoven geotextiles was obtained and compared. The reliability of the low-field NMR measurement results was verified by combining the NMR imaging technology and Python programming pixel recognition method. The results show that the low-field NMR measurement technology with hydrogen atom as probe can accurately determine the effective pore size of nonwoven geotextiles. The combination of the NMR imaging technology and Python programming pixel recognition method can verify the accuracy of the low-field NMR technology in determining the effective pore size of nonwoven geotextiles. The low-field NMR measurement technology can avoid the shortcomings of the wet sieving method and improve the accuracy of the effective pore size measurement results.