for example,my index has 4 rows albums ,I want insert ad between two rows albums?

Count the albums within the next_album loop and insert the ad after the 2nd album.

thank you,but what's the php code?

Set a variable like $count=''; before the loop and then put ´$count++;` within the loop. Then check if $count is 2 and put out your ad. That's it.

This is really basic php basics. You should really consider learn a few things, there are a lot places on the web to do so.