How to send multiple same name input fields value via ajax post method

Try this one

var paidamt = $("input[name=paidamt]").map(function(){
return $(this).val();
}).get().join(",");

var uid = $("input[name=uid]").map(function(){
return $(this).val();
}).get().join(",");


$.ajax(
{
type: "POST",
url: 'paidamt.php',
data:
{
    paidamt:paidamt,
    uid:uid
}
});

Firstly you have given the input elements the same id which is repeated in the loop. This will end up in your HTML being invalid, you should change the id to class:

<form method="post">
    <table border="1">
        <tr>
            <th>Product</th>
            <th>Price</th>
            <th>Paid Amount</th>
            <th>Check</th>
        </tr>
        <?php
            $sql = mysql_query("SELECT * FROM `tbldemo`");
            while ($result = mysql_fetch_array($sql)) { ?>
                <tr>
                    <td><?php echo $result['pname']; ?> </td>
                    <td><?php echo $result['price']; ?></td>
                    <td><input type="text" name="paidamt[]" class="paidamt"></td>
                    <td><input type="checkbox" name="uid[]" class="uid" value="<?php echo $result['id']; ?>"></td>
                </tr>
            <?php }
        ?>
    </table><br>

    <button type="submit" name="submit" id="submit">Save Amt.</button>
</form>

To actually send the input values in the AJAX request you can simply serialize() the containing form when the form is submit:

$(function() {
    $('form').submit(function(e) {
        $.ajax({
            url: "paidamt.php", 
            type: 'POST',
            data: $(this).serialize(),
            success: function(data) {
                $("#divShow").html(data);
            }); 
        });
    });
});

I suggest to add class instead of id, since identically class can be repeated but id should not.

<script type="text/javascript">
function getValue()
{
    var paidamtval = [];
    $('#paidamt').each(function(){
       paidamtval.push($(this).val());
    });
    $.post("paidamt.php",
    {
      paidamt : paidamtval,
      uid : $('#uid').val()
    },
      function( data){
        /*alert(data);*/
        $("#divShow").html(data);
    });  
 }
</script>

Since you will have many of these, id - needs to be unique, which in your case isn't, so remove "id="paidamt"

    <td><input type="text" name="paidamt[]" id="paidamt"></td>

That's your first mistake. And secondly don't use $.post, to submit this form. Either remove AJAX submit, or bind form using something like jQuery Form plugin.

You try this code

$('document').ready(function(){
$('#submit').click(function(){

 jQuery.ajax({
            type: "POST",
            url: "paidamt.php",
            data: new FormData(this),
            contentType: false,       
            cache: false,             
            processData:false,    
            success: function(html){
               try{ 
                 $("#divShow").html(data);
                 }catch (e){
                 alert(JSON.stringify(e));
                 }
             },
                 error : function(e){alert("error "+JSON.stringify(e)); }
        });
        });


        });

in you paidamt.php file

$paidamt=$_POST['paidamt'];// its can array values
print_r($paidamt);// result display