Visitors to Republic of the Congo must obtain a visa from one of the Republic of the Congo diplomatic missions unless they come from one of the visa exempt countries.
Visa exemption
Citizens of the following 13 countries can visit the Republic of the Congo by obtaining a visa on arrival:
Visa not required for citizens of China provided their normal passports are endorsed "For Public Affairs".
Visa not required for holders of a V.I.P Invitation letter.
Holders of diplomatic or service passports issued to nationals of China and Russia do not require a visa as well as holders of diplomatic passports of Portugal.
Transit without visa is allowed for travellers continuing their trip by the same or first connecting plane without leaving the airport.
References
Visa policy of the Republic of the Congo Wikipedia(Text) CC BY-SA