When his car came to a stop, the 37-year-old managed to get out unassisted and was later checked, cleared and released from the Indianapolis Motor Speedway's Infield Medical Center.
"It was understeer on the car, and even if I lifted the throttle on the entry of the corner, it was not enough, and I lost completely the front aero," Alonso said.
"The wall came too close and too quickly. Unfortunately, it happened today. We will lose a little bit of running time again. I'm sorry for the team, but we will learn and hopefully we will come back stronger tomorrow."
Before the crash, Alonso had recorded 46 laps, with a best of 225.433 mph.
Alonso on May 26 will try to join Graham Hill as the second driver to achieve the "Triple Crown of Motorsport" by adding an Indy 500 win to his triumphs at the Monaco Grand Prix and 24 Hours of Le Mans.
It will mark the second time Alonso has taken part in the race, which he led in 2017 before retiring with an engine problem.
(Reporting by Frank Pingue in Toronto, editing by Ed Osmond)
