I have a variable speed drive, the values of the PLC thermostat are all zeros as long as the VSD drive operation, and if the ac drive stops running, the data return to normal immediately (I remove the output lines of the variable speed drive, it runs to 50HZ and the data is normal). Do not know how to deal with it.

I've tried several ways which all were not useful, like ground, separate the VSD drive and control lines, put steel pipes to the speed drives output lines, nothing working, now I'm intending to add the filter, still do not know if there is effect or not, it's a 7.5kw speed drive, I bought a 23A filter. anyone knows how to handle it?

You can try:
First, the connection between the PLC and the thermostat replaced with shielded cable, and it is best if the shield layer single-grounded;
Second, try to add filtering toroid to the variable speed drive output terminals, output filter and output reactor at the drive output.

First of all, it's clear that to make the temperature logging devices work properly, change the ac drive in a big action is unrealistic. Therefore, basically we can do is shielding, filtering, grounding practically.
Second, in strong interference environment, the shield of the transmission line is necessary, but if you do not have rich field experience, the shielding effect is often not very satisfactory.
If only the temperature measuring fluctuations, due to the temperature is slow change, adopt filtering is more efficient, the filter can be a passive RC low-pass filter and connect it to the connection port of the inspection instrument.