The present invention provides an apparatus and a method for manufacturing a CIGS absorber of a thin film solar cell. The apparatus includes a supply chamber configured to provide a flexible substrate coated with precursors. The apparatus further includes a reaction chamber coupled to the supply chamber for at least subjecting the precursors on the flexible substrate to a reactive gas at a first state to form an absorber material. Additionally, the apparatus includes a gas-balancing chamber filled with the reactive gas at a second state. The gas-balancing chamber is communicated with the reaction chamber for automatically updating the first state of the reactive gas to the second state. Moreover, the apparatus includes a control system to maintain the second state of the reactive gas in the gas-balancing chamber at a preset condition and to adjust the transportation of the flexible substrate through the reaction chamber.