The ship-to-ship transfer system using hoses has great advantages because it can effectively deal with the changing distance between vessels. Affected by ocean currents or other factors， the distance between the vessels may increase， which leads to flexible hose being damaged or even broken. To prevent this， the vessel separation detection should be set to monitor the distance between vessels and trigger an alarm when accidents happen. By analyzing the influence of the vessel's movement such as drift between vessels， a calculation method for the safety envelope surface of the ship-to-ship transfer system was proposed. Based on the parameters of a 14000m3 LNG bunkering vessel， the ESD1 and ESD2 alarm distance were calculated as 9.5m and 17m， respectively. The RELATIVE deviations from the corresponding parameters of 9.3m and 16.8m in foreign systems are 2.1% and 1.2%， respectively. It fully proves that this calculation method can provide guideline for the parameter ESD1 alarm setting of vessel separation detection， so as to improve the safety of ship bunkering process.