Park's opponent in the semifinals will be Paula Creamer, who moved on thanks to a 1-up victory over Julieta Granada.
The other semifinal match on Friday will feature Mina Harigae, who birdied the 19th hole to defeat Morgan Pressel, against Sukjin-Lee Wuesthoff, who bested Esther Choe, 3 and 2, in the quarters.
Park tallied five birdies in her first 13 holes to blow out Lu. So far in the match-play portion of this championship, Park, a two-time medalist, has not had a match go past the 14th hole.
'I was hitting my driver far and straight and hit my irons very close and my putting was okay,' said Park, who is looking to become the second player to repeat as Girl's Junior champion. 'If I will defend it, that would be great. I feel a lot better than last year. I have a lot of confidence going into the semifinals.'
Creamer and Granada were all-square headed to the 17th but Granada missed the green short at 17 and Creamer landed in the middle of the green. Granada could not get up and down for par and Creamer sank a two-footer for par to go 1-up with one to play.
At the 18th, Granada once again missed the green and she left with bogey. Creamer also bogeyed the hole but it was enough for a halve and a 1-up victory. It also set up a meeting with the defending champion.
'In-Bee is a great player,' said Creamer, who participated in this year's U.S. Women's Open. 'I definitely didn't play that great. I left a lot of shots out there and missed too many greens. If I play the way I did it's going to be a struggle, but I think I can step it up. It's going to have to be a lot better this afternoon.'
Pressel, who bounced golf phenom Michelle Wie in Thursday's third-round action, and the short-hitting Harigae finished 18 even and headed back to No. 1. Harigae reached the green in regulation for the first time in the championship and made birdie to eliminate Pressel.
'I just play my own game,' said Harigae, who also needed extra holes to beat Brittany Lang in the third round. 'As my dad always told me, 'always be up, never be down.''
Wuesthoff, who played in the ShopRite Classic on the LPGA Tour this summer, won three holes in a row from the 12th to dispatch Choe.
The U.S. Girls' Junior Championship is one of 13 championships conducted annually by the United States Golf Association, 10 of which are strictly for amateurs. The U.S. Girls' Junior is open to female amateur golfers who have not reached their 18th birthday prior to the end of the championship and have a USGA Handicap Index of 18.4 or better.