The way you explain your problem, postreq.params
seems to be a JavaScript variable. If so, this is what you should exec:
var exec = require("child_process").exec;
exec('java -cp %SIKULI_HOME%\sikuli-script.jar org.python.util.jython E:\automation.sikuli\automation.py "' + postreq.params + '"', function (err, data) {
I'm not a Windows guy so I'm not positive that %SIKULI_HOME%
will be expanded like you want. If not, then using process.env.HOMEPATH
like Phoenix suggested in a comment should do the trick:
exec('java -cp ' + proces.env.HOMEPATH + '\sikuli-script.jar org.python.util.jython E:\automation.sikuli\automation.py "' + postreq.params + '"', function (err, data) {