Abstract: With the development of power grid in China, it is an optimized transmission configuration to lay parallel HVAC and HVDC lines in an adjacent corridor or on a same tower. However, the corresponding design standard to limit the hybrid electric field of the transmission lines, which is the basis to design the height of the conductors to ground and the width of the corridor, has not been proposed. The principles to determine the limit of hybrid electric field are discussed and recommended. Then, two types of human perception, head hair perception and spark discharge perception, are measured under hybrid electric field with different DC and AC (power frequency) components. According to the statistical results of the experiment data, the relationship curves between AC (power frequency) electric field and DC electric field are derived when the human sensation is acceptable under some certain probabilities. Based on these curves, the formulas to calculate the design limit of the hybrid electric field are obtained. The results in the research can provide technical support to control the hybrid electric field for design of the parallel HVAC and HVDC transmission lines.