Is there any way to allow user only one product in cart. I used uc_restrict_qty but After that i can add more than one products also.
If the module
uc_restrict_qty not working file your issue there. Alternatively You can achieve this with help of hook hook_uc_add_to_cart
hook_uc_add_to_cart($nid, $qty, $data)
Performs extra processing when an item is added to the shopping cart.
Inside hook you can check your conditions, and prevent your new item to add.
uc_cart_get_contents() can used to get existing contents of cart. If it is greater than 1 you can do your logic here, either remove previous one and add the current one or remove the new item