at first thank you for your help, I have solved the problem.I have seen this example,sinbc()function.and I add a new function in biot.py. but it  
something wrong.
1.I added a new facet named Right and select its region.Like "'Right' : ('vertices in (x > 0.999999)', 'facet'),"
2.a new function added in biot.py. 
3.I load a stress to the left and right facet. if the stress is a constant,it worked.but when I added a new function on the left facet and another new function on the right facet,it didn't work.

r.
在 2015年5月27日星期三 UTC+8下午3:41:59,Robert Cimrman写道:
On 05/26/2015 11:04 AM, Kid Guo wrote:
> Hi  Robert,
> at first thank you for your help, I have solved the problem, and now I want
> to use "sfepy/examples/multi_physics/biot.py", I suspect to add  Dirichlet
> (essential) boundary conditions in the biot. Here I want to fix the bottom
> and load a press on the left facet and right facet, the press can be same
> or different, actually I want more is to load a linear function(like,P = γgh)  from Top
> to Bottom along the z axis. How can I achieve it.

Check [1] - sinbc() function - it shows how to apply Dirichlet BC using a function.

r.

[1] http://sfepy.org/doc-devel/examples/diffusion/sinbc.html