Though impossible to demonstrate without any DDL for the underlying tables it certainly does look like you can convert your function into a Inline Table Valued function (as has been mentioned a few times already). Your function is only hitting three tables (TB_WORK_CALENDAR, TB_CODE_WORK_TIMEDTL, TB_RCV_ALCDATA) and your variables are pulling their values from those tables. You should be able to lose those variables and get this into a single sql statement (perhaps using a CTE) which can be used for an iTVF.

You may also want to take a look at this article: How to Make Scalar UDFs Run Faster (SQL Spackle)[/url]

Note: this is about Scalar Valued Functions but I included it because there is an example of ad-hoc SQL out-performing a function along with a good explanation of why.