The syntax is not correct. The correct expression is [position()=sql:variable("@var")]. Here is an example:

DECLARE @x XML, @cnt INT

SELECT @x = '

<a>

<n>node 1</n>

<n>node 2</n>

<n>node 3</n>

</a>'

SELECT @cnt = 1

WHILE @cnt <= 3 BEGIN

SELECT @x.value('(/a/n[position()=sql:variable("@cnt")])[1]','VARCHAR(20)')

SELECT @cnt = @cnt + 1

END

/*

Prints

node 1

node 2

node 3

*/