However, the government said it would continue with its huge investment programme ahead of hosting the football World Cup in 2022, despite the anticipated deficit.
"The fall in oil prices that began in June 2014 was not anticipated," said Saleh Al-Nabit, development planning and statistics minister.
"If they persist, lower oil prices will narrow the government's fiscal cushion but our considerable financial reserves will provide an ample buffer.
"Important capital spending plans will proceed."
Doha is expected to spend some $200 billion (180 billion euros) on infrastructure projects over the next decade.
These include a new metro system, a new port, and even a new city north of the capital Doha.
The country's controversial hosting of the 2022 tournament, long criticised over human rights and corruption claims, has come under increased scrutiny in the past few weeks.
Following the arrest of FIFA officials in Zurich, Swiss judicial authorities have announced an investigation into the awarding of the 2022 World Cups as part of a far-reaching corruption scandal.
Officials overseeing plans for the tournament say their preparations remain unaffected.