Natural Growth 

You should be mindful of your citizen's Morale as you develop you city. As your city improves, the Morale of the citizens will increase and your population will grow naturally. The formula for determining the maximum value a city's population can reach is: Population Limit x Morale. The population is refreshed every 6 minutes. To check your city’s population details, click on the City Info tab. 

Increase Population Limit：To increase a city's maximum population, you should build and upgrade Houses. 

Improve Morale: Morale determines how quickly your population will increase. Lowering the Tax Rate will slowly increase Morale. You can also go to the Senate and select an event from the Pacify Public section to immediately boost Morale.

Rapid Growth
Quickly increase your city's population. 

You can use the item "Population Boost - 20%," which will increase the city's current population capacity by 20% or 100 (whichever is greater). 

You can also go to the Senate, select Govern -> Pacify Public -> Celebration.