S&P said SriLankan Airlines is 92.1 percent owned by the government of Sri Lanka and state controlled entities hold a further 6.4 percent of shares.
It has run losses for the past five years and has relied of government and other shareholders for support, S&P said.
"The bonds constitute direct, unsubordinated, and unsecured obligations of SriLankan Airlines," S&P said in a statement.
"SriLankan Airlines intends to use the net proceeds from the bonds to finance aircraft pre-delivery payments and for general working capital purposes."
Sri Lankan Airlines has ordered a fleet of new A300 and A350 aircraft from Airbus.