The retailer, which has added 100 brands to its online fashion catalogue, predicts the site will drive fashion sales up by 30% this year.
The improvements include a faster search engine and fashion pages with drop down menus. It will also offer zoomable pictures and colour swatches to show alternative colours and related products.
Robin Terrell, managing director at John Lewis Direct, said: "Fashion online is one of the biggest business opportunities for John Lewis over the next three years and there will be a significant step change in our customers' shopping experience online."
The retailer also began trials of a click and collect service. This allows shoppers to order goods online from John Lewis and pick them up from one of the group's 214 Waitrose supermarkets.
The trial aims to encourage online shopping by giving buyers more options for picking up their goods. John Lewis has 20 outlets in the UK.
John Lewis outlets in Hexham and Tonbridge are taking part in the trail.