Slug flow with varying degrees often occurs in gathering and transportation systems, which makes the flow in the pipelines fluctuate in a certain extent. The fluctuation caused by slug flow made the running of inlet devices in the gathering and transportation system and central treatment station unstable. The flow fluctuation at inlet must be considered when designing inlet devices (usually the primary separator) to inhibit shut down problems due to surpassed flow volume. Suitable design factors were usually considered to relieve the effect caused by flow fluctuation. However, the design factor usually could not solve the unstable problems completely, and dynamic simulation was very necessary. The inlet flow variation, terrain fluctuation and pigging conditions of a gathering and transportation system in a Middle East oilfield were simulated. It was found that the existing separator could guarantee the system operated stable though the inlet flow was fluctuated in a range. The design factor of the inlet primary separator equaled to 1.4 was caused by flow fluctuation.