During harvesting of rapeseed, the falling of siliques and seeds caused by colliding and vibration of the harvester header accounts for about 50% of total harvesting loss (8%~10%). To analyze the impact of vibration of header on the harvesting loss during field operation, an experimental study was conducted on the 4LL-15Y crawler rape combine harvester. Through vibration tests, the key vibration excitation sources were determined, which were the engine (27.53Hz) and the cutter bar (7.93~8.53Hz). Then, the frequency range causing the falling of siliques and seeds was determined using a Default Shaker hydraulic vibration system. The hydraulic shaker generated a frequency sweep from 0Hz to 40Hz on the “Huayouza 62” rapeseed stalks, and maximum falling occurred in the frequency range of 5~10Hz. Singlefactor experiments were conducted between the vibration excitation frequency and the falling of siliques and seeds. Results showed that when the frequency was 7Hz, the stalks had maximum falling of siliques and seeds under both horizontal and vertical fixations. The maximum falling ratio was 3.42% when the stalks were under horizontal fixation. It can be concluded that the reciprocating cutter bar, whose excitation frequency was about 8.0Hz, was the main vibration source causing harvesting losses. The research result can provide some reference for the optimization design of rapeseed and grain combine header cutter. 