他是完全的人，又是完全的神。
如果他是從約瑟和馬利亞肉體所生的後代，按神的定律，必然會帶着原罪。
但是因為他是透過聖靈感孕，神就創造了一個可能，避免了原罪。
而且又成就了一個完全神、完全人的這個我們不可能完全明白的奧秘。

以後再找一些資料來補充。

先看下面這個：

How Did Jesus Avoid Original Sin?
Question

In Sunday school today, someone asked how Jesus could have been born sinless if all fallen humans (including Jesus' mother Mary) were born with original sin? Can you answer that for us?

Answer

It is true that all fallen humans are born with original sin. Original sin consists of the fact that God holds us all guilty for the first sin which Adam committed, our loss of original righteousness, and our consequent moral inability and corruption. Jesus, however, was not a fallen human. The Holy Spirit specially conceived him, so that Jesus was in a very real sense different from the rest of us. Even though he was fully human, his humanity was pure and unfallen, uncorrupted by sin. Moreover, God did not reckon original sin to his account.

But even thought the Bible tells us that Jesus did not inherit original sin (e.g. 2 Cor. 5:21; 1 John 3:5), it does not tell us precisely how he avoided this inheritance. Part of the problem is that the Bible also does not clearly tell us exactly how original sin is communicated to each individual. Some suggest that original sin is communicated from generation to generation, while others suggest that it is communicated directly to each individual by virtue of that person's solidarity with Adam (cf. Rom. 5:12-19). I hold the latter opinion.

Regardless of the specific mechanism by which Jesus avoided original sin, he was only able to do so through the direct and miraculous intervention of God. God is the one who made the agreement with Adam, and who made Adam the representative of all who would come after him, and who laid the curse on Adam and his descendants. Evidently, God did not fully extend Adam's representation of mankind to Jesus, but allowed Jesus some special exclusion.

Perhaps, but not necessarily, he did this by passing the legal representation through the fathers and not through the mothers. The biblical genealogies seem to demonstrate that the promises and obligations of the covenant are passed on through the fathers rather than through the mothers. It is not unreasonable to think that Jesus may have avoided certain aspects of covenant solidarity with Adam (and thus avoided the consequent covenant curse of guilt and pollution from original sin) by being born without a human father. This is not to say that original sin is not communicated directly to each of Adam's posterity, but only that the basis for that communication may be traced genealogically. But again, the Bible does not lay this out explicitly, and it is rather speculative. One significant complication is the fact that in the Bible women also inherit covenant blessings and curses. In short, it's a great question, but the Bible doesn't give us a clear answer. It only assures us of the result: Jesus was born without original sin.

Answer by Ra McLaughlin
http://thirdmill.org/answers/answer.asp/file/99821.qna/category/th/page/questions/site/iiim