您可以尝试两种方式:

DB::table('product_warehouse')
    ->where('product_id', $product_id)
    ->where('warehouse_id', $warehouse_id)
    ->update(['product_qty' => DB::raw('product_qty + ' . $product_qty)]);

或

DB::table('product_warehouse')
    ->where('product_id', $product_id)
    ->where('warehouse_id', $warehouse_id)
    ->increment('product_qty', $product_qty);