The high temperature superconducting (HTS) dynamic synchronous condenser (DSC) attracts close attentions because of excellent reactive compensation characteristics such as fast response time and small synchronous reactance. In this paper, the overall electromagnetic design of a 10 Mvar-class HTS DSC is designed and analysed. The rated terminal voltage and armature current of the air-core water-cooled stator are 11 kV and 525 A, respectively. The excitation winding wound by 10 mm wide REBCO high temperature superconducting material coated conductors, is cooled to below 30 K through helium gas. At rated gap field 1.4 T and no-load excitation current 375 A, the maximum flux density on the REBCO coils is 3.05 T. The analysis of the screening and damping layer shows that it has some conflicted features and needs more accurate design. A simple circuit simulation is carried out to calculate the reactive compensation performance. At the full-load excitation current 428 A, this prototype can output 10 Mvar reactive power.