I'm H1b visa holder. and my fiancee's asylum application in process in Canada. what if she cancel it and return to country to apply for H4 visa?

As your fiancée has not applied for asylum in the US, there should be no impediment to her of obtaining an H-4 visa on that score. Inasmuch as the doctrine of dual intent – a person having both an intent to immigrate and an intent to be in the United States on a nonimmigrant basis – applies to H-1B as well as their dependents, her stay in Canada and not in the home country in the recent past should not affect her ability to obtain the visa. I assume of course that you will be marrying since the H-4 is only for spouses and not fiancées. Your fiancée may wish to bring whatever proof that she has of your bona fide relationship when she applies for the H-4 visa. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.