尝试

$cartHelper = Mage::helper('checkout/cart');
$items = $cartHelper->getCart()->getItems();
foreach ($items as $item) {
    if ($item->getProduct()->getId() == $yourProId) {
        if( $item->getQty() == 1 ){
            $cartHelper->getCart()->removeItem($item->getItemId())->save();
        }
        else if($item->getQty() > 1){
            $item->setQty($item->getQty() - 1)
            $cartHelper->getCart()->save();
        }
        break;
    }
}

看看@ / app / code / core /法师/结帐/控制器/CartController.php

Take a look @ /app/code/core/Mage/Checkout/controllers/CartController.php

请参见 http://www.magentocommerce.com/boards/viewthread/30113/