The Renault works team finished just sixth overall in 2017, and Prost says that the only goal for next year is an improvement.
"I think we have to be realistic and careful with our objectives," the Frenchman told Auto Hebdo.
"Why? Because the teams in front of us have been building up for years. We must evolve step by step but keep showing that we are progressing.
"This is to acknowledge what needs to be done to narrow the gap to Mercedes, Ferrari and Red Bull. What we have to show next year is that instead of being 1.5 seconds behind per lap, it is only 0.8.
"That's how we will do it. We must build, not just the car but a team spirit and attention to detail."
Renault have won two F1 world championships - in 2005 and 2006.