I have done a 2d SPH shear box model in ls-dyna and the result is not good. When the bottom box moving, SPH sample disappeared. could someone help me find the problem out
It's disappearing? PrePost has a setting that will hide SPH that has been deactivated, but if you did not turn it on then that is not likely your problem.
This might sound silly, but have you zoomed out or tried tracking particles when they vanish? SPH sometimes explodes comically catastrophically if the pressure propagation through the medium is not managed properly.
Also, I have never done any of the 2d SPH things, but I remember reading its for problems that are nearly incompressible. I don't know off the top of my head how that formulation communicates between particles, but if its communicating some astronomical force at the moment of deformation it could be sending your SPH into Ludacris speeds.