I am having difficulty to find the English word for this stuff. It is actually a plastic rope that is usually used to measure.
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Most such flexible measuring instruments are called "tape measures" or "measuring tapes". There's a pretty wide variety of them, from the stiffish ones for carpentry or other craftsmanship that use a spring to wind themselves to the more cloth-like kind used for tailoring, which this seems likely to be. Assuming this line has markings on it for distance, that would seem to be the right name for it.
It might be difficult to find a word for it because I'd not look at that and think "measurement". I might think "twine" or "strap" or "string" (though one usually wouldn't use the word "string" for plastic).
As a technique, someone might use a piece of string to see how long something is. Then they might go measure that against something like a ruler or "yardstick". And as @NathanTuggy points out there are various kinds of measuring tape.
(At Ikea, they give out free paper rulers, for measuring furniture in the store.)
But there is no special type of unmarked measuring string that I know of with a name in English. Anything without marking would just be plain string or twine, and anything with markings would be a "measuring tape".