Abstract: During the decommissioning of nuclear facility, it is necessary to accurately quantify the radioactivity of large-area transuranic nuclide planes over 1 000 cm². Common non-destructive measurement methods have small effective measurement area and high detection limit, which cannot meet the requirement of rapid analysis of large-area low-activity sample demand. The grid ionization chamber is an instrument used for low-level alpha radioactivity spectrum measurement, which can meet the analysis requirement of low-level sample. However, due to the electrode processing technology, the measurable sample is generally less than 500 cm². To solve this problem, an ultra-large area grid ionization chamber with a sensitive area close to 2 000 cm2 was developed. Through the adjustment and optimization of the pressure and voltage, the ultra-large area grid ionization chamber has a 1.8% energy resolution for the ²⁴¹Am electrode position source, a minimum detectable activity of 10^-2 Bq, and a 2π angle detection efficiency of 97.7% for a flat sample with a diameter less than 460 mm.