Step 1 .Click on Settings > Filters > Create a new Filter
Step 2 .You would see 4 text boxes. In “Has the words” enter is:spam and click on next step .
Step 3. Click OK on the pop up that follows.
Step 4. Check the box against “Never send it to Spam” and click on “Create filter”
http://www.aeinrst.info/turn-off-or-disable-gmail-spam-filter/