The invention provides a natural gas transportation pipeline top pressure power generation system based on a single-screw expander, and belongs to the technical field of waste heat and excess pressure energy utilization. The natural gas transportation pipeline top pressure power generation system is connected to an existing natural gas conveying pipeline in an adjacent mode to replace a pressure reduction valve set (1) of the existing natural gas conveying pipeline. High-pressure natural gas is expanded and does work through the single-screw expander (2). An electric generator (4) is driven by a movable speed reducer (3). The expanded low-pressure natural gas enters an oil-gas separator (5) to separate natural gas from lubricating oil. The natural gas which is separated out enters the existing natural gas conveying pipeline through an air heater (6) and is conveyed to a downstream user. The lubricating oil which is separated out is pumped into the position, needing to be lubricated, of the single-screw expander again after being pressurized through a lubricating oil pump (7). The natural gas transportation pipeline top pressure power generation system is characterized in that electricity generation is conducted through pressure energy of the natural gas conveying pipeline by means of the single-screw expander, and meanwhile low-temperature natural gas is heated back to the temperature required by the downstream user through the air heater.