[en] The increase in the BBU threshold current is very important for the high and medium energy electron linacs because of a larger operating current attainable or a smaller emittance available at certain operating current. For this purpose, many means can be used, among which the improvement in the accelerating structure itself is always fundamental. SLAC's three-meter long section is unique with truly constant gradient performance. The theoretical analyses and operating experiences at SLAC have indicated that detuning the Nos. 3, 4 and 5 cavities of some sections by 2 or 4 MHz for TM₀₁ wave had considerably raised the BBU threshold current. A method of opening four holes symmetrically distributed on disks as described in detail will get the benefits in improvement of BBU threshold current