how to set parent process datafield to ipc datafield

When you create the IPC, you can specify parameters to be 'return' parameters - map these to the parent data field and then, when they are set in the child process, the value will be returned to the parent process.  I hope this is what you were looking for, let me know if you need more details.

when i designed workflow useing vs2005,i had setted mapping fields from parent to child.and debuged the parent code,the normal,but child process can not received value.

Just a not on this, if you start the child process Asynchronously (i.e. The parent starts the child and continues), you cannot pass information back from child to parent. This is only possible if you start the process Synchronously, where the parent will wait for the child to complete before continuing. 

but my aim is to pass value  to child-processes,but can not

You have to manually create the data fields in the child (IPC process), so that you can map the data in the parent data fields to the child data fields when you run the IPC event wizard.