Speaking in Moscow before leaving, he said the crisis could not be allowed to “drag on”, but said any timetable required agreement from all sides.
Russia has offered support for his plan, which proposes a UN-supervised ceasefire and political dialogue.
Correspondents say Moscow’s backing may make Mr Annan’s job in Beijing easier.
Russia and China were heavily criticised after vetoing two UN Security Council resolutions condemning the violence, which the UN says has left more than 8,000 people dead.
After meeting the former UN secretary general on Sunday, Russian President Dmitry Medvedev said his country fully supported the mission because it might be “Syria’s last chance to avoid a protracted and bloody civil war”.
Chinese officials have said they hope the talks will find a solution to the conflict.
“China values and supports the mediation efforts of Mr Annan and hopes this visit will allow in-depth discussions on a political resolution of the Syrian issue,” foreign ministry spokesman Hong Lei said on Monday.
Meanwhile, Syria’s divided opposition groups are meeting in Istanbul to try to achieve a more united front, ahead of a second “Friends of Syria” meeting on 1 April.
The main opposition coalition, the Syrian National Council (SNC), said the aim was to produce “the common objectives of the Syrian opposition to end the regime’s dictatorship and realise the final objective of establishing a pluralist civil and democratic state”.
The BBC’s Jonathan Head in Istanbul says they need a credible and coherent opposition movement if they are to achieve more than the previous meeting in Tunis, which was widely viewed as a failure.
Mr Annan’s plan calls for President Bashar al-Assad to pull troops and heavy weapons out of restive cities, and for all parties to allow for a daily two-hour pause to the fighting in order for humanitarian aid to reach affected areas.
It also requests that the Syrian authorities release those detained in the uprising.
However it does not impose any deadline for Mr Assad, or call for him to leave power.
Earlier Mr Annan’s spokesman also said the Syrian government had replied to his six-point peace plan and that he would “respond very shortly”.
On Monday, activists said security forces had again been bombarding the central city of Homs. The Syrian Observatory for Human Rights said at least five civilians had been killed by mortar fire in the northern district of Khaldiya.
Restrictions have also been placed on men of military age – 18 to 42 – trying to leave Syria, officials told the Reuters news agency. They must now get permission from military recruitment and immigration departments before travelling.