The umqtt.simple module is sync.You'd better use peterhinch's uasync mqtt lib.

While you only need subscribe to some topic.
my solution is :

• use check_msg() instead of wait_msg(),cause check_msg() return immediately.

import uasyncio
from umqtt.simple import MQTTClient

loop = uasyncio.get_event_loop()
async def my_callback(topic,msg):
   await uasyncio.sleep(1)
   
def sub_callback(topic, msg):
   global loop
   loop.create_task(my_callback(topic,msg))

async def my_main():
   c = MQTTClient("umqtt_client", server)
   c.set_callback(sub_callback)
   c.connect()
   c.subscribe(b'foo_con')
   pass

loop.creat_task(my_main())
loop.run_forever()

@2niuhe thanks, but using umqtt_async have its own problem : https://forum.micropython.org/viewtopic.php?f=15&t=7144

thanks for your solution but i had problem in publish part not others .