In order to deal with the gradual shortage of labour force in the aging society, it is necessary to make intelligent and unmanned improvements to mechanical equipment such as combine harvesters for repetitive daily work. Based on a special combine harvester, the electrical modification for its reaping unit is designed, which includes the selection of motor and reducer, the 3D design and finite element analysis for two kinds of working conditions. Moreover, the electrical driver method is proposed, which can automatic adjustment harvest speed based on the walking speed of vehicle. Finally, some filed tests of wheat harvesting verified the correctness for electric refit. The electrical modification of reaping unit provides theoretical basis and experimental data for the robotics research of whole combine harvester.