A Stephen Cole goal gave Ireland an early lead which they held until the end of the first quarter.
However, second-quarter goals from Simon Gougnard and Cedric Charlier helped Belgium lead 3-1 at half-time.
With Tom Boom notching a hat-trick, Belgium moved 6-1 ahead before Alan Sothern's late reply.
Ireland keeper David Harte made a series of brilliant saves - including taking a point-blank shot on his mask in the third quarter as the Olympic silver medallists produced relentless pressure.
"Belgium are a good team and have got a lot of power in a lot of areas," said Ireland coach Craig Fulton after the game in Johannesburg.
"Their corners are a strength so when you give away soft ones, it does hurt you. We had a good few phases but couldn't sustain it for longer periods."
Despite the loss, Ireland are next in action against Egypt on Thursday where a win will assure Fulton's side of a place in the quarter-finals and a big step toward securing a 2018 World Cup ticket.
"It's about the next game and there are some areas to improve on but I am really looking forward to the next phase," Fulton added.
The Irish will qualify for the 2018 World Cup if they finish in the top five in the 10-team tournament in Johannesburg.
Ireland, who defeated hosts South Africa 2-0 in their opener on Sunday, will also play Egypt and Germany in Group B.
The Irish side went into the tournament as the fifth highest ranked of the competing nations.
Ireland squad: David Harte (GK), Jamie Carr (GK), John Jackson, Jonathan Bell, Matthew Bell, Chris Cargo, Matthew Nelson, Alan Sothern, Eugene Magee, Neal Glassey, Shane O Donoghue, Sean Murray, John McKee, Paul Gleghorne, Jeremy Duncan, Conor Harte, Stuart Loughrey, Stephen Cole.
Ireland's hopes of qualifying for next year's World Cup suffered a setback as they were beaten 6-2 by Belgium at the World League semi-finals tournament.