The seventeenth century`s literatures with Mongolian letter had ceased their historical life for recent 80 years because of political reason, and new orthography did not match with morphological feature of Mongolian language. Therefore, to restore original meaning of a lot of lost vocabularies we should rely on the contemporary materials. When studying the seventeenth century`s Mongolian language, it is possible to make use of Todo-letter, Soyomboletter, and Pagspa-letter beside Mongolian letter. In addition, the study on the seventeenth century`s Mongolian language involves an extended problem on contemporary phonetics, morphology, style, grammar, speech and dialect.