The invention discloses a single preheating and heat accumulating condensation energy-saving boiler using low-calorie fuels, comprising a furnace body. Two or more burners are arranged on the furnace body; a high-temperature combustion-air inlet and a fuel inlet are arranged on each burner; the high-temperature combustion-air inlet is connected with a regenerator; the regenerator is connected with a constant-temperature combustion-air inlet and a low-temperature fume outlet; a switching-over unit is arranged between the constant-temperature combustion-air inlet and the low-temperature fume outlet; and the low-temperature fume outlet is connected with a condensation heat recoverer. Firstly, during smoke venting, the heat-energy loss is reduced and the combustion efficiency is improved by two stages of waste-heat recovery through the regenerator and the condensation heat recoveror; secondly, the constant-temperature combustion-air participates in the combustion-supporting after being preheated by the regenerator, and as a result, the combustion temperature is increased; thirdly, a plurality of burners adopt periodical switching-over combustion, thereby, local high-temperature area inside the combustion chamber is avoided, the distribution of temperature in the combustion chamber is more even, and the service life of the furnace chamber is prolonged. The invention is applicable to various gas boilers.