Because of the difficulty to fabricate the ultra-thin mirror individually, we proposed a base supporting method. Methods which were applied to control the distortion caused by adhesion, heat, and stress during ultra-thin mirror fabricating were researched, and means to estimate the effect was put forward. Technology of controlling mirror distortion was improved based on the experience on φ195 mm ultra-thin mirror manufacture. Then a φ340 mm ultra-thin mirror was fabricated by improved technics. When it was de-blocked from blocking body, the surface error of PV=5.74λ, RMS=1.02λ (λ=632.8 nm) caused by distortion is acceptable. Those experiments indicate that those technics can control distortion effectively and technical improvement is correct. Results of these experiments also indicate that the final error on ultra-thin mirror surface is astigmatism, which is not caused by gravity.