Lori Loughlin vehemently denies her daughter was filming herself behind the wheel moments before she got into a car crash ... which was posted on YouTube, but has since been taken down.

The "Fuller House" star was at LAX when we asked about allegations her 17-year-old daughter, YouTube star Olivia Jade Giannulli, was vlogging while driving ... capturing the moment a truck rear-ended her, shoving her into another car.

Olivia reportedly posted the video Sunday to her nearly 1 million subscribers. Daily Mail posted 6 screen grabs showing her behind the wheel, and reported Olivia vowed to never vlog or text while driving right after the accident.

Mama Bear says the report is total BS, and insists her daughter never recorded while driving. It's impossible to compare her explanation though .. Olivia's video has been taken down from YouTube, for some reason.