Lou was relieved of his duties and replaced by 59-year-old Xiao Jie, the deputy secretary-general of the State Council, or cabinet, Xinhua news agency reported without giving more details.
Lou, 65, had been the finance minister since March 2013, presiding over a stock market boom encouraged by authorities -- followed by a rout in the middle of last year.
Citic Bank International chief economist Liao Qun told AFP that Lou's replacement could be routine due to his age, but that the new finance minister could raise market expectations of tax cuts to spur the economy.
But he added, "A new head could mean fiscal policies could play bigger roles in boosting growth and China could be more aggressive in fiscal policies, given it has already used monetary tools many times."
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China's economy grew an annualised 6.7 percent in July-September, the same as the first two quarters.
The government has targeted 6.5-7.0 percent growth for the year, following 6.9 percent last year -- the slowest rate in a quarter of a century.
In 2015 China cut its interest rate five times and several times lowered the amount of cash banks must set aside. It also cut the reserve rate once this year.
Other analysts argued that the new minister would mean little difference.
Raymond Yeung, chief greater China economist at ANZ Banking Group, told Bloomberg News, "This personnel change is not expected to change China's fiscal policy.
"The government is still aiming to support growth through a deficit budget of 3 per cent of GDP. Since Xiao has a good understanding of the tax system, the appointment can facilitate the continuity of tax reform."
Lou, who is on the board of governors of the China-backed Asian Infrastructure Investment Bank, has been one of best-known names representing China's economic views alongside central bank chief Zhou Xiaochuan.