Abstract: Friction and frictional torque are main restriction factors for horizontal displacement of long reach well. The optimized build up section can reduce both friction and frictional torque. The optimization of build up section was studied and a new design was proposed for a new trajectory of long reach well, i. e. side catenary trajectory. The calculation analysis showed that the side catenary trajectory has the shortest wellbore and minimum friction and frictional torque compared with the circular and hypobolic trajectories, which needn’t any circular transition section and facilitate design and operation. This paper is instructive for field design and operation by providing all the calculation formulas as well as calculation examples.