I used a physical material, the weight of transparency is 0.5, and it looks translucent in 3dsmax viewport, but after I exported it as a gltf model using babylon plugin, it becomes opaque in gltf viewer.
Adding @Guillaume_Pelletier our 3dsmax expert 
at the bottom of the Material property panel, into Slate Editor, you might select Blend for the transparency mode (which is opaque by default)
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It works, thank you very much 
Cool