Abstract: The principle of measuring the bunch length of the Beijing electron positron collider upgrade(BEPCⅡ) linac by harmonic bunch length monitor is presented. The measured bunch length ranging between 1.4 mm and 2.0 mm agrees well with the theoretic calculation of 1.5 mm. The electron beam current measured by the beam current monitor (BCT3) is almost in direct proportion to the induced voltage of the fundamental harmonic. When the parameters of the pre-buncher and buncher are adjusted greatly while the beam current being unchanged, the fifth harmonic signal has a great change, but the fundamental signal changes gently. The test results are all in good agreement with the theoretic analysis.