The invention relates to a portable sulfur trioxide sampling apparatus and method based on a controlled condensing method. A snake-like condenser tube of the apparatus is arranged in a temperature-controlled box, wherein an upper end of the snake-like condenser tube is connected to an air inlet pipe, and the lower end of the snake-like condenser tube is connected to a gas-liquid separator. The gas-liquid separator is connected to a liquid sample excretion pump and an air exhaust pipe. One end of an absorption liquid duct is located in an absorption liquid storage container, and the other end of the absorption liquid duct is connected to the upper end of the snake-like condenser tube. The absorption liquid duct is equipped with an absorption liquid supplying pump. An air exhaust pipe solenoid valve and an electronic flowmeter are arranged on the air exhaust pipe. The electronic flowmeter, the air inlet pipe solenoid valve, an air suction solenoid valve, the liquid sample excretion pump and the absorption liquid supplying pump are connected to a single-chip microcomputer. The portable sulfur trioxide sampling apparatus is high in automation degree and measurement precision.