Although Bohai oilfield is still the main force of China offshore oilfield, some blocks had run out of production and need to be decommissioned. NB field which belonged to CNPC was in this state and has been decommission. In this job, most of procedure had been optimized. For pursued the best cutting performance, Abrasive Waterjet Cutting System (AWCS) had been used for multi-layer casing-strings and platforms cutting operation. Resistance force of casing-string pulled out and platform lifted had been calculated by using soil mechanics for choosing the suitable floating crane before the lifting operation. A numerical simulation was provided for checking the max displacement of platform after pile legs were cut. Moving sequence had been made based on the results of the simulation. The operation time of single casing-string had been reduced by using optimized working flow. With assistant of 150 tons class floating crane, 7-8 hours could be saved for each casing-string. Assemblage and dismantlement operation of casing-strings and pile legs cutting except the first and last casing-string or pile leg of the platform could be saved. The results of numerical simulation showed that platforms would have enough stability for decommission operation while all the pile legs had been cut. Based on the simulation results, all the platforms had been moved used one-stop operation after they had been cut and much time and money was saved. Based on the results of calculation of pulling out resistance force, 150 tons class and 500 tons class floating crane had been chosen for lifting casing-strings and platforms. During the job, the deviation of casing-strings was bigger than pile legs. The extra pulling loading was appeared for conductors. In order to find out the reason of that phenomenon, an experiment was conducted. The results of experiment could become a guidance for the next job.