Katie Walsh's mount Battlefront, trained by her father Ted, collapsed and died during the Fox Hunter's Chase after being pulled up at the 11th fence.
Katie Walsh, who hopes to become the first woman to win the Grand National on Saturday, tweeted of her sadness.
"Very sad to lose Battlefront today. We had many great days and he was a great teacher. He was a gent and I will miss him very much," she said.
Her father had said earlier in the week that horses were treated "better than some children".
The Fox Hunter's is considered the biggest race for amateur jockeys.
"I would like to extend our sympathies to the Walsh family following the sad news," said John Baker, Aintree and North West Regional director for Jockey Club Racecourses.
"You can never remove risk from all horse racing, as with any sport. However, welfare standards are very high [in Britain] and equine fatalities are rare."
The race was surprisingly won by 100-1 shot Tartan Snow.
Fourteen of the 25 starters in the race finished.
It came on the same day that top trainer Jonjo O'Neill hit out at animals rights activists for trying to get the Grand National banned, telling them that they "just don't understand".
Two horses have died in each of the last two years at the race, leading to increasingly loud calls from campaigners to put an end to the world's most famous National Hunt event.
O'Neill knows only too well about the regular fatalities at Aintree: last year he watched in horror as his horse Synchronised, who had won the Cheltenham Gold Cup the previous month, suffered a fatal fall.
But the Irishman told The Times that those who criticise British racing's showpiece event do not know what they are talking about.
"It's always been a fantastic race and it still is," said O'Neill, who trained 2010 National winner Don't Push It.
"People saying the National should be stopped just don't understand.
"We are all in racing because we love the horses, but we take things to heart.
"Some of the things said about us are a bloody insult, as if we are animals ourselves.
"There are people out there stabbing and shooting, killing randomly, and sometimes it seems we are being put in the same bracket."
Recent deaths have prompted several changes to the course, and there will be more this year as the start has been moved forward 90 yards while timber frames in the fences have been removed.
But the changes have always seemingly been to little avail: safety improvements began in earnest after the 1989 race saw two horses killed, yet since then horses have continued to die in the race at an average of one a year.
Jockeys in the Fox Hunters' Chase were positive about the changes.
Jamie Hamilton, 18, won the race on Tartan Snow and said: "It's the first time I have ridden over them but they seemed spot on."
Sam Waley-Cohen, who rode Cottage Oak, told the BBC: "They are still big obstacles and are a test for horse and jockey."
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