After several days I finally learned that the Flex ASM could just worked in a cluster.
When I first got this feature, I tried to build two servers, one is DB server, and another is ASM server, and want the DB could able to connect the ASM server. I didn't know how to build them, but the test did attrack me.
I have to read more, when I read the ASM administration guide the second time, I got that only hub node could access the ASM server, even in Flex ASM mode, and from the related pictures I found more clear explanation: the ASM metadata was transferred through, but the actual disk IO isn't.
Yes, this is the point, even in Flex ASM mode, the DB server also need to access the shared disk directly! So all the leaf nodes are not able to access the ASM nodes.