Abstract: This paper establishes the equations of the allowable velocity of pipe trip based on power law fluid in different-working conditions，under the condition that the effective bottom hole pressure remains greater than or equal to formation pressure during the pulling of the pipe out of the hole,and that the effective liquid column in the hole in leakage zones remain small-er than fracture pressure during the running of the pipe into the hole.

Key words: Balance pressure drilling, Blow out, Lost circulation, Power law fluid, Pipe trip, Surge pressure