Display all employee
I developed a query,but it fetching those record where asd.sal_ID is
null,but i want to fetch all employee from tbl_Employee_Master
table,when ever this query is run'

The asd.sal_ID IS NULL criteria in the WHERE clause will exclude all employees with non-null deductions. Move that criteria the the ON clause of the allowance deduction LEFT JOIN to include allowance deduction rows with NULL Sal_ID values without excluding employees with non-null values too:
SELECT
      M.Emp_ID
    , M.EMp_Name 
    , M.EMP_Salary 
    , D.Designation_Name AS Designation
    , SUM(CASE T.DA_Name  WHEN 'Loan' THEN (ASD.assign_amt) ELSE 0 END) Loan
    , SUM(CASE T.DA_Name  WHEN 'Advance' THEN (ASD.assign_amt) ELSE 0 END) Advance
    , SUM(CASE T.DA_Name  WHEN 'Mobile' THEN (ASD.assign_amt) ELSE 0 END) Mobile
    , SUM(CASE T.DA_Name  WHEN 'Conveyance' THEN (ASD.assign_amt) ELSE 0 END) Conveyance
    , SUM(CASE T.DA_Name  WHEN 'Medical' THEN (ASD.assign_amt) ELSE 0 END) Medical
    , SUM(CASE T.DA_Name  WHEN 'Rent' THEN (ASD.assign_amt) ELSE 0 END) House_Rent
    , ISNULL(M.emp_OT,0) AS emp_OT
    , 0 AS 'txtpresent'
    , 0 AS 'txtabsent'
    , 0 AS 'txtsalary'
    , 0 AS 'lbOT_Amount'
    , 0 AS 'txthour'
    , ASD.Sal_ID
FROM #tbl_Employee_Master M
INNER JOIN #tbl_designation D ON D.Designation_ID=M.Designation_ID
LEFT JOIN #tbl_assign_Allowance_Deduction ASD ON asd.Emp_ID=M.Emp_ID AND ASD.sal_ID IS NULL
LEFT JOIN #tbl_Type_allowance_Deduction T ON T.DA_ID=ASD.DA_ID
GROUP BY
      M.Emp_ID
    , M.EMp_Name
    , M.EMP_Salary
    , D.Designation_Name
    , M.emp_OT
    , ASD.Sal_ID;