The present invention relates to morphinan derivatives and preparation methods thereof, especially to ketal hydroxyl protected compounds of morphinan derivatives and preparation method thereof, and to a method for preparing corresponding alkylated morphinan derivatives by using the ketal hydroxyl protected compounds as intermediates, and more especially to a ketal hydroxyl protected compound of methylnaltrexone as intermediate for preparing methylnaltrexone and a method for preparing methylnaltrexone through said intermediate.