This is my problem and I hope finding a solution.
_In the simplest alternative, MSTJ(22) = 2, the comparison is based on the average lifetime, or rather (c*tau "time life") , measured in mm. Thus if the limit PARJ(71) is (the default) 10 mm, then decays of K0_S, Lambda, Sigma−, Sigma+, Theta−, Theta+, Theta0 and Omega− are all switched off, but charm and bottom still decay._
This is what I found in the manual of PYTHIA, I did this but the decay of b is not happened.
Knowing that the decay of b is activated by putting "MDCY(PYCOMP(5),1)=1".