The three-day congress will be held in Bulawayo, the country's second-largest city dominated by the minority Ndebele who have roundly rejected Mugabe in every election for the last decade.
No date has been set for elections to choose a government that will replace the transitional power-sharing pact created after 2008 polls collapsed in a deadly spiral of violence that left more than 200 of Tsvangirai's supporters dead, according to rights groups.
Mugabe's Zanu-PF has already endorsed the 87-year-old as its candidate, and the aging leader will use the congress as a platform to push for elections next year, said independent political analyst Charles Mangongera.
"Looking into the conference, Mugabe wants an election next year, although that may not happen," Mangongera said.
Mugabe had pushed for elections in 2011, but in a sign of his more limited power under the unity accord, Tsvangirai rebuffed his demands with regional backing.
The unity deal calls for a new constitution to be approved by referendum, a process running more than a year behind schedule with no firm indication of when it might be complete.
In the meantime, Mugabe will continue to play Zanu-PF's powerful factions off against each other, Mangongera said.
"Zanu-PF is a divided organisation, it has various factions driven by certain interests both political and financial," Mangongera said.
One camp is led by Defence Minister Emmerson Mnangagwa, a hardliner reviled in Bulawayo for his role in the 1980s massacres of Ndebeles that left 20 000 dead.
The other camp was led by the late general Solomon Mujuru, who died in a mysterious fire in August. His wife, Vice President Joice Mujuru, is still rallying supporters in the wake of his death.
"Mujuru was a kingmaker. His departure will be felt as he was the only person who could face the old man [Mugabe] to say that this was wrong or right," said Takavafira Zhou, a political scientist at Masvingo State University.
He said the Mujuru faction "may use the conference to mobilise and emerge stronger".
Mugabe has for decades used party divisions to his advantage, arguing that without his leadership, Zanu-PF would "implode", he added.
In power since independence from Britain in 1980, Mugabe faces another challenge in worries about his health.
Long-whispered rumours burst into the open with US diplomatic cables released this year by the whistleblower website WikiLeaks, in which top Zanu-PF officials told American diplomats that Mugabe has prostate cancer that has spread through his body.
Mugabe has made near monthly visits to Singapore all year, although he firmly denies that he's receiving major medical treatment there.
Party spokesperson Rugare Gumbo told state media last week that the congress would "want to study what the WikiLeaks are all about, are they genuine or meant to destroy the party".
In typical fashion, most of the party's power struggles will play out behind closed doors. The public comments will likely focus on calls for elections and on pushing forward a new law requiring foreign firms to cede majority stakes to local blacks.
"The conference will be stage-managed for the different factions who will be preparing for the post-Mugabe era," Zhou said. "They will not discuss critical issues like the succession issue."