WINDSOR, Ontario (AP) _ Retailers in this Canadian border city expect their registers will be stuffed with American dollars through Jan. 1, as Michigan shoppers seeking a better deal creep through the tunnel from Detroit.

''All you have to do is check the parking lots,'' said Sgt. Edward Jee, a Windsor police officer who patroled the downtown streets here Thursday. ''It's two out of three American.

Windsor merchants and their American customers are reaping a holiday shopping bonanza because the U.S. dollar, worth 35 percent to 40 percent more than its Canadian equivalent, buys more across the border than at home.

''It creates quite a backlog at the tunnel,'' said Jee. ''but merchants don't complain and I don't either.''

Americans peering into store windows along Windsor's Oulette Avenue on Thursday, which wasn't a holiday in Canada, that the favorable exchange rate drew them along with the prospect of not being jammed into suburban Detroit shopping malls with millions of other frantic shoppers.

Others said a trip to Windsor is simply a treat.

''We live on campus at Wayne State University, it's a pretty depressed area,'' said Virginia Souza, 40, of Detroit. ''The transportation system in Detroit is terrible. It's worse than that, it's non-existent. This is civilization for us.''

Her husband, Daniel, 41, a doctoral candidate at Wayne State, said, ''The Canadian dollar is a great advantage, but I like the atmosphere, too.''

Americans are permitted to return home with up to $25 of merchandise duty free, if the shopper was in Canada less than 48 hours. If the goods exceed $25, everything is subject to duty. American Customs officials collected $170,000 in December 1984, said Jack Vaughn, head of the tunnel's U.S. Customs station.

Aside from the relative values of their currency, Windsor proprietors say, there's not much difference between American and Candian consumers. But an employee at Beldore Jewelry Inc., said Michigan shoppers often are aggressive.

''They really know what they want,'' said Barb, who declined to give her last name. ''This year I've been making diamonds - big diamonds. They buy easier than the Canadians. They're used to spending money.''

Nicholas Karataglidis, 23, helps run the chic D&K Furriers International on Oulette Avenue. Sitting among minks, rabbits and other luxurious coats, he said November and December should be the best selling months of 1985.

''The markets are primarily supported by Americans,'' he said.

Audre Hanes, executive director of Windsor's Downtown Business Association, said, ''I think Americans are recognizing the quality of Canadian fashions and they seem to enjoy the variety of restaurants we have here.''

A family from Monroe, about 20 miles south of Detroit, walked through Windsor with their hands full of Christmas gifts. Bernard Beneteau, 53, said he treated his wife and daughter to Thanksgiving dinner at a local hotel, and bought a few presents, as well.

''I come over here about three or four times a year,'' said Beneteau, who owns a jewelry shop. ''You can't beat the exchange rate and the satisfaction that your money goes a long way.''