Since MP arms needed to be folded and the motors are closer to body,how do you calibrate the IMU w/o Magnetic Interference?

You can't, but it isn't a problem for the IMU calibration and you can just ignore the magnetic interference message warning you get during the calibration.

I see where your coming from and it’s a bit of an anomaly, IMU does contain magnetometers, but it works with its arms folded so should be no problem.

IMU needs to be operated when the drone is folded. And like answers above, you can ignore the interference when doing the IMU calibration.

I did calibration just like that and its says "IMU calibration success" but after re start it says calibration failed on the sensor state window... My mavic stays on the ground now. It worked fine before upgrade. It also lags very much ( before IMU error) , What can i do?!?

DJI Thor Posted at 2017-11-21 00:46
IMU needs to be operated when the drone is folded. And like answers above, you can ignore the interference when doing the IMU calibration.

That is the missing part of that IMU Calibration Tutorial......users should be warned that "Magnetic Interference Error" will be present during the procedure and still be "OKAY" to proceed.

fans8958c37f Posted at 2017-11-21 01:12
I did calibration just like that and its says "IMU calibration success" but after re start it says calibration failed on the sensor state window... My mavic stays on the ground now. It worked fine before upgrade. It also lags very much ( before IMU error) , What can i do?!?

Please help to make a screenshot of the IMU info and firmware info, we would like to check it first.
Here's the route:
IMU: connect the drone, go fly, upper right icon, MC Settings, Advance Settings, Sensor.
Version: connect the drone, go fly, upper right icon, General Settings, About.
Also, please help to confirm if there is a crash before.
And for the transmission issue, may I know what mobile device and app's version you are using?