Abstract: The selection of neutral grounding mode is a basic problem in distribution systems.For active distribution networks（ADNs）containing a large amount of distributed generators（DGs）,the neutral grounding mode will be decided by both main network and DGs and the single-phase earth fault current and overvoltage will be more complex.So the neutral grounding mode of ADNs needs to be re-examined.Possible grounding mode combinations of main network and DGs are given out.Grounding mode transformation problems,single-phase earth fault current and overvoltage problems in different operating states and different stages of single-phase earth fault handling process including pre-and post-line protection of main network or islanding protection,are analyzed.Then the basic idea about grounding mode selection of DGs for each existing distribution network is proposed,whose neutral grounding modes are determined,and about grounding mode selection of a brand new ADN,without considering all the constraints of insulation and relay protection conditions in existing distribution networks.A preliminary study on neutral grounding mode of ADNs is made.