Abstract: According to no-pressure pipeline hydraulic calculation theory，and based on the critical slope gradient and the
critical unsliting speed，the paper presents the design method and rules of slope gradient and the size of horizon tank to settle
such issues as the unsteady level of mud in the return channel and the under change of mud height，which hinders the early
kick detection while drilling. With the calculation of examples，the slope gradient is optimized and the size of horizon tank is
modified to make sure of the steady level of mud，whitout talus accumulation. Besides，the fluid level sensitivity is analyzed
before and after the modification，and the results show that when the change of flow is small，the fluid level of tank changes
obviously. It is recommended to use supersonic kick detection with high accuracy to detect the change of level，and finally
detect the kick with pit level method. It has been proved to be of advisability with field experiment.

Key words: kick, return flow, fluid level, slope gradient, horizon tank size