Buffering with fiber delay line(FDL) has emerged as the current time-domain content solution for variable length optical packet switching(OPS).By calculating the packet loss ratio(PLR) of OPS with FDL buffering,we find the PLR degrade in variable length switching when the traffic load is high(i.e.,the traffic load is larger than 0.7).Then,we study the hybrid buffer structure which consists of the conventional FDL buffer as the prime buffer and a shared electronic buffer as the supplementary buffer.We put forward an improved first fit void filling(IFF-VF) algorithm for the hybrid buffer structure to improve the PLR.The results of the analysis and the simulation experiments showed the use of hybrid buffer together with the IFF-VF algorithm can greatly improve the variable length optical switch performance in terms of PLR and decrease the FDL number when the traffic load is high.