how can i measure the trip point accurately?

hi, in design of comp with hysteresis, for example 40mv,
how can i measure the hysteresis accurately?

add a sawtooth, pay attention to that the period is longer

You can build a simple device which will precisely allow you to measure both trip points.

Construct multistage binary counter, for example 16 stages. Connect identical resistors between every output and 1 common point. This common point can be directly connected to tested comparator's input, or, better option, use an opamp voltage follower as a buffer ..
Connect LED diode (with current limiting resistor) to the output of the comparator under test and 0V..

Now, assuming that you have 5V circuit, 16-bits give you 5/65536 precision, ≈75µV resolution.
If you need better resolution add next 2, 4 or as many stages as you wish ..

If you start countig UP you will find 1st trip point; next count DOWN and you will find 2nd trip point; knowing both counts you can easily calculate your histeresis ..

Regards,
IanP