To fully utilize the voltage source characteristics of grid forming control and coordinate multiple reactive power sources within renewable energy power plants to enhance their transient voltage support capability in power systems with high renewable penetration, a decentralized coordinated transient voltage control strategy is proposed for renewable energy plants based on grid forming converters. This strategy includes an improved transient voltage control strategy for multiple reactive power sources and a method for capacity allocation. First, the basic characteristics of reactive power sources in the power plant are analyzed. Then, a transient control based on reactive power compensation is introduced into grid forming converters to increase the transient internal potential voltage. Grid following converter and static var generator (SVG) respectively adopt adaptive transient active/reactive control and reference value adjustment strategies to increase their transient reactive power outputs. For static var compensator (SVC), a transient switching strategy is employed to switch from steady-state reactive power control to voltage control to further improve voltage regulation performance. Meanwhile, a capacity allocation method for multiple reactive power sources is proposed based on maximum transient current and reactive compensation requirement. Finally, a typical scenario of active transient voltage support is built under weak network conditions to validate that the proposed decentralized collaborative control strategy improves the transient voltage support capability of the plant, highlighting the role of grid forming control in active transient voltage support.