In recent years, the holographic antenna (HA) composed of impedance unit cells fed by surface waves has been developed rapidly. However, the cylindrical conformal holographic antenna (CCHA) is recently designed with planar unit cell impedance, which will result in beam deterioration and loss of gain and efficiency. Therefore, a way to calculate the impedance of curved unit cells is presented in this letter. Compared with the planar impedance calculation (PIC), the efficiency of the HA using the curved impedance calculation (CIC) is increased from 17.05% to 19.13% with the fixed deflection angle θ of 0° when the radius of the cylinder is 18 mm. An approximate calculation for the impedance of arbitrary cylindrical radius is proposed based on the PIC and CIC, which can achieve a more convenient calculation of impedance without significantly affecting performance. It has been shown that the efficiency of the HA using the approximate impedance calculation (AIC) is increased from 17.05% to 19.42% under the same conditions compared with the HA using the planar impedance calculation. The efficiency can also reach about 20% in the case of a small radius and angle less than 20 °, which is close to the efficiency of planar HA. The measured radiation efficiency is 21.48% with a small radius. The low efficiency of the CCHA has been improved.