Resolution:
When you implement a Web Service server side using http in BW, you can not direct access the http-related properties either from SOAP event source, or Service activity. Actually these properties can be obtained using Http Receiver activity as a “Web-service proxy”. For this situation, there is a workaround that http receiver receives soap request and gets http header, then redirect soap request to Webservice; I attached a sample project for reference, whose flow is described below:
1.    The client process sends a SOAP message (which contains “SOAP request”) to the port 8002.
2.    The ProcessProxy process receive the SOAP message at port 8002, and then redirect the SOAP message to port 8001 using Send Http request activity.
3.    The server process receives the SOAP message at port 8001 using SOAP event source activity.
4.    The sever process send SOAP response message, and the send Http response get this response message from the output of the send Http request activity.
5.    The client process receives the response SOAP message (which contains “SOAP response”), which is sent by send Http response activity in ProcessProxy process.
In this scenario, you can acquire the value of http properties (such as “content-type”) in the output of Http receiver activity.

How to obtain Remote IP Address through Load Balancer when BW Soap RR calls a WebService over http?