Marriage and Your Credit Score
Your spouse's bad history won't necessarily tarnish yours.
Will my credit score go down if I marry someone with a bad score?
Not necessarily. There are no joint credit reports or scores, so getting married in itself won't lower your score. But becoming a co-signer on an account with a bad history will tarnish your record (for more about the pitfalls of co-signing, see Say No to Co-Signing).
Lenders will look at both credit histories if you apply for a loan together. And the bad one could carry more weight. See if you can qualify for the loan with only one income, or wait to apply until your spouse's score improves. For advice on improving your credit score, see Demystifying Your Credit Score.
See also Marriage and Money 101 for more information about couples and finances.