Field practice is an important part of training geography research talents. However, traditional teaching methods may not adequately manage, share and implement instruction resources and thus may limit the instructor's ability to conduct field instruction. A possible answer is found in the rapid development of computer-assisted instruction (CAI), a new teaching mode for the Information Age. A "virtual field trip" is an amalgam of Internet and multimedia techniques with great potential for geography field instruction, as it can arouse students' enthusiasm and immerse them in learning. However, limited by available technology, virtual field trips have disadvantages in terms of content presentation, data organisation, and loss or inadequate representation of spatial information. To overcome the above shortcomings, a map-based, spatially correlated design method was proposed for a CAI system in geography. By integrating spatial information technology (Geographic Information System, Global Positioning System, remote sensing, etc) and information delivery methods (web services, databases, etc), a WebGIS-based teaching assistant system for geography field practice (TASGFP) was established, realising an effective spatial management scheme and forming a shared platform for instruction material. Its efficiency was verified through the actual application, although there remains room for improvement. Nevertheless, the TASGFP stands as an effective CAI tool in geography instruction.